Teach scanner to respect the working directory set in the invocation
through scanner C API.
Also add test infrastructure to testing scanner from C API. Break up
DependencyScan lib into two so the swift-scan-test and remain small
without understanding swift AST.
rdar://127626011
This does not seem to serve a purpose other than corrupting arguments with whitespaces - they get merged into one large string where the whitespace boundary between arguments and whitespaces within arguments are blurred.
Part of rdar://98985453
For the client library (`libSwiftDriver`), this is critical for multiplexing library instances, and for compatibility with different versions of `libSwiftDrvier`.
Resolves rdar://73631930
Adds a C API layer consisting of:
- Data structures used to represent in-memory result of dependency scanning
- Opaque dependency scanner tool (C wrapper for `DependencyScanningTool`)
Refactors `ScanDependencies.cpp` to produce dependency scanning result in the form of the above binary format.
