[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

include/swift-c/DependencyScan/DependencyScan.h

@@ -25,7 +25,7 @@
 /// SWIFTSCAN_VERSION_MINOR should increase when there are API additions.
 /// SWIFTSCAN_VERSION_MAJOR is intended for "major" source/ABI breaking changes.
 #define SWIFTSCAN_VERSION_MAJOR 0
-#define SWIFTSCAN_VERSION_MINOR 6
+#define SWIFTSCAN_VERSION_MINOR 7
 
 SWIFTSCAN_BEGIN_DECLS
 
@@ -505,10 +505,26 @@ SWIFTSCAN_PUBLIC void swiftscan_cas_dispose(swiftscan_cas_t cas);
 /// swift input on the command-line by convention. Return \c CacheKey as string.
 /// If error happens, the error message is returned via `error` parameter, and
 /// caller needs to free the error message via `swiftscan_string_dispose`.
+/// This API is DEPRECATED and in favor of using
+/// `swiftscan_cache_compute_key_from_input_index`.
 SWIFTSCAN_PUBLIC swiftscan_string_ref_t
 swiftscan_cache_compute_key(swiftscan_cas_t cas, int argc, const char **argv,
                             const char *input, swiftscan_string_ref_t *error);
 
+/// Compute \c CacheKey for the outputs of a primary input file from a compiler
+/// invocation with command-line \c argc and \c argv and the index for the
+/// input. The index of the input is computed from the position of the input
+/// file from all input files. When primary input file is not available for
+/// compilation, e.g., using WMO, primary file is the first swift input on the
+/// command-line by convention. Return \c CacheKey as string. If error happens,
+/// the error message is returned via `error` parameter, and caller needs to
+/// free the error message via `swiftscan_string_dispose`.
+SWIFTSCAN_PUBLIC swiftscan_string_ref_t
+swiftscan_cache_compute_key_from_input_index(swiftscan_cas_t cas, int argc,
+                                             const char **argv,
+                                             unsigned input_index,
+                                             swiftscan_string_ref_t *error);
+
 /// Query the result of the compilation using the output cache key. \c globally
 /// suggests if the lookup should check remote cache if such operation exists.
 /// Returns the cached compilation of the result if found, or nullptr if output