[clang][cli] NFC: Ensure non-null DiagnosticsEngine in ParseDiagnosticArgs

Before this patch, ParseDiagnosticArgs can be called with a nullptr DiagnosticsEngine *. This happens early on in the compilation process, where no proper DiagnosticEngine exists, because the diagnostic options (passed through command line) are not known yet.

This patch ensures nullptr is replaced by an ignoring DiagnosticEngine in ParseDiagnosticArgs, which allows to switch from pointer to a reference in some utility functions.

Besides simplifying the code, this patch enables a future patch (D84673) that ports diagnostic options to the new marshalling infrastructure.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D93701