[GWP-ASan] Add GWP_ASAN_ prefix to macros.
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.
Reviewed By: eugenis
Subscribers: #sanitizers, llvm-commits, cferris, pcc
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D70683