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.
Switch to use clang-include-tree by default for clang module
building/caching when using a CAS. This is the default mode for clang
module and has less issues than CAS file system based implementation.
Allow DependencyScanner to canonicalize path using a prefix map. When
option `-scanner-prefix-map` option is used, dependency scanner will
remap all the input paths in following:
* all the paths in the CAS file system or clang include tree
* all the paths related to input on the command-line returned by scanner
This allows all the input paths to be canonicalized so cache key can be
computed reguardless of the exact on disk path.
The sourceFile field is not remapped so build system can track the exact
file as on the local file system.
