Bump DIAG_SIZE_SEMA, as we've hit it.

$ grep 'DIAG_SIZE_SEMA          =' include/clang/Basic/DiagnosticIDs.h
      DIAG_SIZE_SEMA          = 4000,
$ grep DIAG $(build)/tools/clang/include/clang/Basic/DiagnosticSemaKinds.inc | wc -l
3499

llvm-svn: 359702