Add support for the readnever concept

The purpose of this concept is to turn the load of debugging
information off, either globally (via the '--readnever' option), or
objfile-specific.  The implementation proposed here is an extension of
the patch distributed with Fedora GDB; looking at the Fedora patch
itself and the history, one can see some reasons why it was never
resubmitted:

  - The patch appears to have been introduced as a workaround, at
    least initially;
  - The patch is far from perfect, as it simply shunts the load of
    DWARF debugging information, without really worrying about the
    other debug format.
  - Who really does non-symbolic debugging anyways?

One use of this feature is when a user simply wants to do the
following sequence: attach, dump core, detach.  Loading the debugging
information in this case is an unnecessary cause of delay.

This patch expands the version shipped with Fedora GDB in order to
make the feature available for all the debuginfo backends, not only
for DWARF.  It also implements a per-objfile flag which can be
activated by using the "-readnever" command when using the
'add-symbol-file' or 'symbol-file' commands.

It's also worth mentioning that this patch tests whether GDB correctly
fails to initialize if both '--readnow' and '--readnever' options are
passed.

Tested on the BuildBot.

gdb/ChangeLog:

2017-12-01  Andrew Cagney  <cagney@redhat.com>
	    Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* NEWS (Changes since GDB 8.0: Mention new '--readnever'
	feature.
	* coffread.c (coff_symfile_read): Do not map over sections with
	'coff_locate_sections' if readnever is on.
	* dwarf2read.c (dwarf2_has_info): Return 0 if
	readnever is on.
	* elfread.c (elf_symfile_read): Do not map over sections with
	'elf_locate_sections' if readnever is on.
	* main.c (validate_readnow_readnever): New function.
	(captured_main_1): Add support for --readnever.
	(print_gdb_help): Document --readnever.
	* objfile-flags.h (enum objfile_flag) <OBJF_READNEVER>: New
	flag.
	* symfile.c (readnever_symbol_files): New global.
	(symbol_file_add_with_addrs): Set 'OBJF_READNEVER' when
	'READNEVER_SYMBOL_FILES' is set.
	(validate_readnow_readnever): New function.
	(symbol_file_command): Handle '-readnever' option.
	Call 'validate_readnow_readnever'.
	(add_symbol_file_command): Handle '-readnever' option.
	Call 'validate_readnow_readnever'.
	(_initialize_symfile): Document new '-readnever' option for
	both 'symbol-file' and 'add-symbol-file' commands.
	* top.h (readnever_symbol_files): New extern global.
	* xcoffread.c (xcoff_initial_scan): Do not read debug
	information if readnever is on.

gdb/doc/ChangeLog:

2017-12-01  Andrew Cagney  <cagney@redhat.com>
	    Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (File Options): Document --readnever.
	(Commands to Specify Files): Likewise, for 'symbol-file' and
	'add-symbol-file'.

gdb/testsuite/ChangeLog:

2017-12-01  Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.base/readnever.c, gdb.base/readnever.exp: New files.

2 files changed, 28 insertions, 1 deletions

diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 6d23a773e9..fa8180fc94 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,11 @@
+2017-12-01  Andrew Cagney  <cagney@redhat.com>
+	    Joel Brobecker  <brobecker@adacore.com>
+	    Sergio Durigan Junior  <sergiodj@redhat.com>
+
+	* gdb.texinfo (File Options): Document --readnever.
+	(Commands to Specify Files): Likewise, for 'symbol-file' and
+	'add-symbol-file'.
+
 2017-11-29  Pedro Alves  <palves@redhat.com>
 
 	PR c++/19436
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 4f552225aa..005fed89c5 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -1037,6 +1037,16 @@ Read each symbol file's entire symbol table immediately, rather than
 the default, which is to read it incrementally as it is needed.
 This makes startup slower, but makes future operations faster.
 
+@item --readnever
+@anchor{--readnever}
+@cindex @code{--readnever}, command-line option
+Do not read each symbol file's symbolic debug information.  This makes
+startup faster but at the expense of not being able to perform
+symbolic debugging.  DWARF unwind information is also not read,
+meaning backtraces may become incomplete or inaccurate.  One use of
+this is when a user simply wants to do the following sequence: attach,
+dump core, detach.  Loading the debugging information in this case is
+an unnecessary cause of delay.
 @end table
 
 @node Mode Options
@@ -18576,6 +18586,15 @@ tables by using the @samp{-readnow} option with any of the commands that
 load symbol table information, if you want to be sure @value{GDBN} has the
 entire symbol table available.
 
+@cindex @code{-readnever}, option for symbol-file command
+@cindex never read symbols
+@cindex symbols, never read
+@item symbol-file @r{[} -readnever @r{]} @var{filename}
+@itemx file @r{[} -readnever @r{]} @var{filename}
+You can instruct @value{GDBN} to never read the symbolic information
+contained in @var{filename} by using the @samp{-readnever} option.
+@xref{--readnever}.
+
 @c FIXME: for now no mention of directories, since this seems to be in
 @c flux.  13mar1992 status is that in theory GDB would look either in
 @c current dir or in same dir as myprog; but issues like competing
@@ -18604,7 +18623,7 @@ the program is running.  To do this, use the @code{kill} command
 @kindex add-symbol-file
 @cindex dynamic linking
 @item add-symbol-file @var{filename} @var{address}
-@itemx add-symbol-file @var{filename} @var{address} @r{[} -readnow @r{]}
+@itemx add-symbol-file @var{filename} @var{address} @r{[} -readnow @r{|} -readnever @r{]}
 @itemx add-symbol-file @var{filename} @var{address} -s @var{section} @var{address} @dots{}
 The @code{add-symbol-file} command reads additional symbol table
 information from the file @var{filename}.  You would use this command