Improve diagnostics message for swift caching build by trying to emit
the diagnostics early when there is more context to differentiate the
different kind of problems.
After the improvement, CAS Error should be more closer to when there is
functional problem with the CAS, rather than mixing in other kinds of
problem (like scanning dependency failures) when operating with a CAS.
rdar://145676736
Fully support make-style `.d` dependencies file output by making
following improvements:
* All correct dependency file render when cache hit for a different
output file location. The dependency file should list the correct
output path, not the stale output path for the initial compilation
* When enable a path prefix mapper to canonicalize the path, the
dependency file should render the input file correctly as the input
file path on disk.
rdar://132250067
