[clang] Give better message for unsupported no_sanitize on globals

Previously if you specified no_sanitize("known_sanitizer") on a global you
would yield a misleading error "'no_sanitize' attribute only applies to
functions and methods", but no_sanitize("unknown") would simply be a warning,
"unknown sanitizer 'unknown' ignored". This changes the former to a warning
"'no_sanitize' attribute argument not supported for globals: known_sanitizer".

Differential Revision: https://reviews.llvm.org/D133117