The MSVC ABI requires that we generate an alias for the vtable which
means looking through a GlobalAlias which cannot be overridden improves
our ability to devirtualize.
Found while investigating PR20801.
Patch by Andrew Zhogin!
Differential Revision: http://reviews.llvm.org/D11306
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242955 91177308-0d34-0410-b5e6-96231b3b80d8
