[Caching] Downgrade output loading error to a warning

If the output loading failed after cache key lookup, treat that as a
warning and resume as if that is a cache miss. This is not a valid
configuration for builtin CAS but can happen for a remote CAS service
that failed to serve the output. Instead of failing, we should continue
to compile to avoid disruptive failures.

rdar://140822432

Author: cachemeifyoucan

include/swift/AST/DiagnosticsFrontend.def

@@ -511,6 +511,7 @@ REMARK(output_cache_miss, none, "cache miss for input file '%0': key '%1'", (Str
 
 // CAS related diagnostics
 ERROR(error_cas, none, "CAS error encountered: %0", (StringRef))
+WARNING(cache_replay_failed, none, "cache replay failed: %0", (StringRef))
 
 ERROR(error_failed_cached_diag, none, "failed to serialize cached diagnostics: %0", (StringRef))
 ERROR(error_replay_cached_diag, none, "failed to replay cached diagnostics: %0", (StringRef))

lib/Frontend/CachingUtils.cpp

@@ -226,7 +226,8 @@ bool replayCachedCompilerOutputs(
                 OutputEntry{OutputPath->second, OutID, Kind, Input, *Proxy});
           return Error::success();
         })) {
-      Diag.diagnose(SourceLoc(), diag::error_cas, toString(std::move(Err)));
+      Diag.diagnose(SourceLoc(), diag::cache_replay_failed,
+                    toString(std::move(Err)));
       return lookupFailed();
     }
   };