[clang] Don't use -Wno-nested-anon-types on GCC (#95029)

GCC usually doesn't warn about unrecognized -Wno-<foo> options, if no
diagnostics are printed. However if some diagnostics are printed, it
also mentions that there were unrecognized -Wno-<foo> options.

Before 4feae05c, we checked for whether
-Wnested-anon-types was supported, and added the -Wno-<foo> form if the
positive form of the option was supported.

As of GCC 14, -Wnested-anon-types isn't supported, thus limit the use of
the option to actual Clang (and still only while using the GCC
compatible driver).

This avoids unnecessary mentions about unrecognized -Wno-<foo> options
when building with GCC.