When caching build is enabled, teach dependency scanner to report
command-lines with `-direct-clang-cc1-module-build` so the later
compilation can instantiate clang importer with cc1 args directly. This
avoids running clang driver code, which might involve file system
lookups, which are the file deps that are not captured and might result
in different compilation mode.
rdar://119275464
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.
Improve tests to call python script through python executable. Even this
doesn't cause any errors now, it will be a problem for future when we
run CAS tests on Windows platform. Clean up the current test to avoid
bad test example.
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
