[GMR] Fix a long-standing bug in GlobalsModRef where it failed to clear

[GMR] Fix a long-standing bug in GlobalsModRef where it failed to clear
out the per-function modref data structures when functions were deleted
or when globals were deleted.

I don't actually know how the global deletion side of this bug hasn't
been hit before, but for the other it just-so-happens that functions
aren't likely to be deleted in the particular part of the LTO pipeline
where we currently enable GMR, so we got lucky.

With this patch, I can self-host with GMR enabled in the normal pass
pipeline!

I was a bit concerned about the compile-time impact of this chang, which
is part of what motivated my prior string of patches to make the
per-function datastructure very dense and fast to walk. With those
changes in place, I can't measure a significant compile time difference
(the difference is around 0.1% which is *way* below the noise) before
and after this patch when building a linked bitcode for all of Clang.

Differential Revision: http://reviews.llvm.org/D11453

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243385 91177308-0d34-0410-b5e6-96231b3b80d8