[Caching] Use clang to prefix-map -fmodule-file-cache-key paths

When prefix mapping paths that are used in clang, ensure we are
consistently using the same prefix mapper from clang. This prevents
mismatches that could cause modules to fail to load.

rdar://123324072

include/swift/DependencyScan/SerializedModuleDependencyCacheFormat.h

@@ -39,7 +39,8 @@ using llvm::BCVBR;
 
 /// Every .moddepcache file begins with these 4 bytes, for easy identification.
 const unsigned char MODULE_DEPENDENCY_CACHE_FORMAT_SIGNATURE[] = {'I', 'M', 'D','C'};
-const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MAJOR = 5; // optionalModuleImports
+const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MAJOR =
+    6; // mappedPCMPath
 /// Increment this on every change.
 const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MINOR = 1;
 
@@ -182,6 +183,7 @@ using SwiftPlaceholderModuleDetailsLayout =
 using ClangModuleDetailsLayout =
     BCRecordLayout<CLANG_MODULE_DETAILS_NODE, // ID
                    FileIDField,               // pcmOutputPath
+                   FileIDField,               // mappedPCMPath
                    FileIDField,               // moduleMapPath
                    ContextHashIDField,        // contextHash
                    FlagIDArrayIDField,        // commandLine