[Caching] Re-associate diagnostics cache key with InputFile

Change how cached diagnostics are stored inside the CAS. It used to be
stored as a standalone entry for a frontend invocation in the cache and
now it is switched to be associated with input files, stored together
with other outputs like object files, etc.

This enables cleaner Cache Replay APIs and future cached diagnostics
that can be splitted up by file contribution.

include/swift/Frontend/FrontendInputsAndOutputs.h

@@ -157,6 +157,8 @@ public:
   /// instead of just answering "batch" if there is more than one primary.
   std::string getStatsFileMangledInputName() const;
 
+  const InputFile &getFirstOutputProducingInput() const;
+
   bool isInputPrimary(StringRef file) const;
 
   unsigned numberOfPrimaryInputsEndingWith(StringRef extension) const;