To allow prefix mapping of the bridging header to achieve cache hit when
source files are located in different location, the generated chained
bridging header should not include absolute paths of the headers. Fix
the problem by concat the chained bridging header together.
Fixes: https://github.com/swiftlang/swift/issues/84088
Make sure the sources for bridging header is added as part of the CAS
filesystem for the main module. Even bridging header should be compiled
into PCH, PCHs are not standalone that it can be used without source
file.
Teach swift dependency scanner to use CAS to capture the full dependencies for a build and construct build commands with immutable inputs from CAS.
This allows swift compilation caching using CAS.
