[C99] Claim conformance to "additional predefined macro names"

This feature is listed in the C99 front matter as one of the changes to
C99, but there was no associated document with that title (or anything
close to it).

I eventually tracked this down to a new requirement in C99 that
implementations defining their own pedefined macros should not name
them with a __STDC_ prefix. Clang does not do this, but there's no
tests that would demonstrate we properly "implement" this feature. So
claiming full conformance without additional test coverage.