[Basic] Stop using a SmallVector<> for Diagnostic. This drops Clang binary size

by ~%.3/~100k in my build -- simply by eliminating the horrible code bloat coming
from the .clear() of the SmallVector<FixItHint>, which does a std::~string, etc.
 - My understanding is we don't ever emit arbitrary numbers of fixits, so I just
   moved us to using a statically sized array like we do for arguments and
   ranges.

llvm-svn: 152639