Currently, swift-frontend will always try to infer a default CASPath if
no `-cas-path` is passed. The function `getDefaultOnDiskCASPath` called
to infer the default path can fatal error in some environment where no
caching directory and home directory, no matter if caching is used or
not.
Remove the inferred path during parsing since that is not strictly
necessary. If caching is enabled and no CASPath is passed, error can be
issued during CAS construction time.
rdar://158899187
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.
Rename `-enable-cas` to `-compile-cache-job` to align with clang option
names and promote that to a new driver only flag.
Few other additions to driver flag for caching behaviors:
* `-compile-cache-remarks`: now cache hit/miss remarks are guarded behind
this flag
* `-compile-cache-skip`: skip replaying from the cache. Useful as a
debugging tool to do the compilation using CAS inputs even the output
is a hit from the cache.
