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

llvm-svn: 247724