[Caching] Encoding cache key for input file with index instead of path

Avoid path encoding difference (for example, real_path vs. path from
symlink) by eliminating the path from cache key. Cache key is now
encoded with the index of the input file from all the input files from
the command-line, reguardless if those inputs will produce output or
not. This is to ensure stable ordering even the batching is different.

Add a new cache computation API that is preferred for using input index
directly. Old API for cache key is deprecated but still updated to
fallback to real_path comparsion if needed.

As a result of swift scan API change, rename the feature in JSON file to
avoid version confusion between swift-driver and libSwiftScan.

rdar://119387650

lib/Frontend/FrontendInputsAndOutputs.cpp

@@ -159,6 +159,10 @@ FrontendInputsAndOutputs::getFirstOutputProducingInput() const {
   return isWholeModule() ? firstInput() : firstPrimaryInput();
 }
 
+unsigned FrontendInputsAndOutputs::getIndexOfFirstOutputProducingInput() const {
+  return isWholeModule() ? 0 : PrimaryInputsInOrder[0];
+}
+
 bool FrontendInputsAndOutputs::isInputPrimary(StringRef file) const {
   return primaryInputNamed(file) != nullptr;
 }