[codeview] Fix symbol names for dynamic initializers and atexit stubs

Summary:
Add a new variant to GlobalDecl for these so that we can detect them
more easily during debug info emission and handle them appropriately.

Reviewers: rsmith, rjmccall, jyu2

Subscribers: aprantl, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60930

llvm-svn: 359148