[clang][tooling] Properly initialize DiagnosticsEngine for cc1 command-line construction

In `ToolInvocation::run`, the driver -> cc1 command-line transformation uses `DiagnosticsEngine` that wasn't completely initialized. This patch ensures `ProcessWarningOptions(DiagnosticsEngine&, const DiagnosticOptions &)` is called.

Depends on D108982.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D108974