[GWP-ASan] Add GWP_ASAN_ prefix to macros.

Summary:
When platforms use their own `LIKELY()` definitions, it can be quite
troublesome to ensure they don't conflict with the GWP-ASan internal
definitions. Just force the GWP_ASAN_ prefix to help this issue.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: #sanitizers, llvm-commits, cferris, pcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D70683