Added llvm.module flag for strict vtable pointers

It is dangerous to do LTO on code with strict-vtable-pointers, because
one module has invariant.group.barriers, and the other one not.

In the future I want to just strip all invariant.group metadata from
vptrs loads/stores and get rid of invariant.group.barrier calls.

http://reviews.llvm.org/D12580

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247724 91177308-0d34-0410-b5e6-96231b3b80d8