This allows the clang driver to know which tool is meant to be executed,
which allows the clang driver to load the right clang config files, and
allows clang to find colocated sysroots.
This makes sure that doing `clang-scan-deps -- <tool> ...` looks up
things in the same way as if one just would execute `<tool> ...`, when
`<tool>` isn't an absolute or relative path.
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
