[llvm-tblgen] Avoid StringMatcher for GCC and MS builtin names

This brings the compile time of Function.cpp from ~40s down to ~4s for
me locally. It also shaves off about 400KB of object file size in a
release+asserts build.

I also realized that the AMDGPU backend does not have any GCC builtin
names to match, so the extra lookup was a no-op. I removed it to silence
a zero-length string table array warning. There should be no functional
change here.

This change really ends the story of PR11951.

llvm-svn: 258897