compiler-clang.h: Add __diag infrastructure for clang

Add __diag macros similar to those in compiler-gcc.h, so that warnings
that need to be adjusted for specific cases but not globally can be
ignored when building with clang.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220304224645.3677453-6-memxor@gmail.com

[ Kartikeya: wrote commit message ]