Add the ShadowCallStack pass

Summary:
The ShadowCallStack pass instruments functions marked with the
shadowcallstack attribute. The instrumented prolog saves the return
address to [gs:offset] where offset is stored and updated in [gs:0].
The instrumented epilog loads/updates the return address from [gs:0]
and checks that it matches the return address on the stack before
returning.

Reviewers: pcc, vitalybuka

Reviewed By: pcc

Subscribers: cryptoad, eugenis, craig.topper, mgorny, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D44802

llvm-svn: 329139