Update swift cache key computation mechanism from one cache key per
output, to one cache key per primary input file (for all outputs that
associated with that input).
The new schema allows fewer cache lookups while still preserving most of
the flexibility for batch mode and incremental mode.
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.
Teach swift how to serialize its input into CAS to create a cache key
for compiler outputs. To compute the cache key for the output, it first
needs to compute a base-key for the compiler invocation. The base key is
computed from: swift compiler version and the command-line arguments for
the invocation.
Each compiler output from swift will gets its own key. The key for the
output is computed from: the base key for the compiler invocation + the
primary input for the output + the output type.
