[AA] Enhance the new AliasAnalysis infrastructure with an optional

"external" AA wrapper pass.

This is a generic hook that can be used to thread custom code into the
primary AAResultsWrapperPass for the legacy pass manager in order to
allow it to merge external AA results into the AA results it is
building. It does this by threading in a raw callback and so it is
*very* powerful and should serve almost any use case I have come up with
for extending the set of alias analyses used. The only thing not well
supported here is using a *different order* of alias analyses. That form
of extension *is* supportable with the new pass manager, and I can make
the callback structure here more elaborate to support it in the legacy
pass manager if this is a critical use case that people are already
depending on, but the only use cases I have heard of thus far should be
reasonably satisfied by this simpler extension mechanism.

It is hard to test this using normal facilities (the built-in AAs don't
use this for obvious reasons) so I've written a fairly extensive set of
custom passes in the alias analysis unit test that should be an
excellent test case because it models the out-of-tree users: it adds
a totally custom AA to the system. This should also serve as
a reasonably good example and guide for out-of-tree users to follow in
order to rig up their existing alias analyses.

No support in opt for commandline control is provided here however. I'm
really unhappy with the kind of contortions that would be required to
support that. It would fully re-introduce the analysis group
self-recursion kind of patterns. =/

I've heard from out-of-tree users that this will unblock their use cases
with extending AAs on top of the new infrastructure and let us retain
the new analysis-group-free-world.

Differential Revision: http://reviews.llvm.org/D13418

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250894 91177308-0d34-0410-b5e6-96231b3b80d8

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