[TableGen] Report an error message on a missing comma

I recently forgot a comma in a defm argument list and tablegen just
failed with exit code 1 without printing an error message. I believe
this issue was introduced in a9fc44c5.

This change prints the following instead:
.../clang/include/clang/Driver/Options.td:569:3: error: Expected comma before next argument

Reviewed By: Paul-C-Anagnostopoulos

Differential Revision: https://reviews.llvm.org/D100178