[Caching] Add fast swift instance setup for cache replay
Add a fast path to create swift CompilerInstance when it is only used to
replay output when there is a cache hit. The normal `setup` function is
very expensive to call, especially in cache mode to setup inputs, and it
needs to be called once per input file from libSwiftScan API due to the
current caching granularity.
The fast path will only construct the part that is needed for output
replay, including the CAS, the output backend and caching diagnostic
processor.
rdar://127062609
