Add ability to automatically chaining the bridging headers discovered from all
dependencies module when doing swift caching build. This will eliminate all
implicit bridging header imports from the build and make the bridging header
importing behavior much more reliable, while keep the compatibility at maximum.
For example, if the current module A depends on module B and C, and both B and
C are binary modules that uses bridging header, when building module A,
dependency scanner will construct a new header that chains three bridging
headers together with the option to build a PCH from it. This will make all
importing errors more obvious while improving the performance.
Improve caching related tests to make them faster and test the situation
closer to what actually happens during explicit module build. Most
noticable changes are:
* Avoid swift stdlib dependency during the tests to save time
* Use dependency scanner output to construct test cases
* Update old test cases that try to simulate caching from file system
inputs to using only CAS inputs.
Redirecting file system can canonicalize the file path before forwarding
the path to IncludeTreeFileSystem, which is a simplied FS that can only
intepret the paths that has been seen by dep-scanner. Since all files
that need redirecting already added to underlying FS via DepScan, there
is no need for such layer when compiling using clang-include-tree.
rdar://119727344
