Add support for __declspec(guard(nocf))

Summary:
Avoid using the nocf_check attribute with Control Flow Guard. Instead, use a
new "guard_nocf" function attribute to indicate that checks should not be
added on indirect calls within that function. Add support for
__declspec(guard(nocf)) following the same syntax as MSVC.

Reviewers: rnk, dmajor, pcc, hans, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: aaron.ballman, tomrittervg, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D72167