[clang][deps] Prevent emitting diagnostics outside of source file

The dependency scanner needs to report the module map file describing the module whose implementation is being compiled (see D134222). However, calling `Preprocessor::getCurrentModuleImplementation()` in the scanner might cause a diagnostic during module map parsing and emitting a diagnostic without being "in" a source file is illegal (e.g. in `TextDiagnosticPrinter`). This patch ensures the module map parse is triggered while the compiler is still "in" a source file, avoiding the failure case.

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D135637