[clang][tooling] Create SourceManager for DiagnosticsEngine before command-line parsing

In D84673, we started using `DiagnosticsEngine` during command-line parsing in more contexts.

When using `ToolInvocation`, a custom `DiagnosticsConsumer` can be specified and it might expect `SourceManager` to be present on the emitted diagnostics.

This patch ensures the `SourceManager` is set up in such scenarios.

Test authored by Jordan Rupprecht.

Reviewed By: rupprecht

Differential Revision: https://reviews.llvm.org/D99414