Allow Compiler.h to be included in C files and fix fallthrough warnings

Summary:
Since clang does not support comment style fallthrough annotations
these should be switched to macros defined in Compiler.h. This
requires some fixing to Compiler.h.

Original patch: https://reviews.llvm.org/D66487

Reviewers: nickdesaulniers, aaron.ballman, xbolva00, rsmith

Reviewed By: nickdesaulniers, aaron.ballman, rsmith

Subscribers: rsmith, sfertile, ormris, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66609

llvm-svn: 369782