[AliasSetTracker] Correctly handle changing the size of an entry

If the size of an AST entry changes, we also need to make sure we perform
necessary alias set merges, as the new size may overlap pointers in other sets.
We happen to run into this with memset, because memset allows an entry for a
i8* pointer to have a decidedly non-i8 size.

This fixes PR27262.

Differential Revision: http://reviews.llvm.org/D18939

llvm-svn: 266381