[clang][cli] NFC: Use Diags to report parsing success/failure

`Success` is set to `false` whenever `Diags.Report(diag::err_)` is called. Remove the duplication and use `Diags` as the source of truth when deciding whether to report parsing success/failure.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D100644