[CodeGenCXX] Emit strip.invariant.group with -fstrict-vtable-pointers

Summary:
Emmiting new intrinsic that strips invariant.groups to make
devirtulization sound, as described in RFC: Devirtualization v2.

Reviewers: rjmccall, rsmith, amharc, kuhar

Subscribers: llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D47299



Co-authored-by: Krzysztof Pszeniczny <krzysztof.pszeniczny@gmail.com>
llvm-svn: 336137