Support hermetic indexing information

Swiftc port of https://github.com/apple/llvm-project/pull/4207.

This introduces a new flag, `-file-prefix-map` which can be used
instead of the existing `-debug-prefix-map` and `-coverage-prefix-map`
flags, and also remaps paths in index information currently.

lib/FrontendTool/FrontendTool.cpp

@@ -1745,7 +1745,8 @@ static void emitIndexDataForSourceFile(SourceFile *PrimarySourceFile,
                                  opts.IndexStorePath, opts.IndexSystemModules,
                                  opts.IndexIgnoreStdlib, isDebugCompilation,
                                  Invocation.getTargetTriple(),
-                                 *Instance.getDependencyTracker());
+                                 *Instance.getDependencyTracker(),
+                                 Invocation.getIRGenOptions().FilePrefixMap);
   } else {
     std::string moduleToken =
         Invocation.getModuleOutputPathForAtMostOnePrimary();
@@ -1760,7 +1761,8 @@ static void emitIndexDataForSourceFile(SourceFile *PrimarySourceFile,
                                  opts.IndexIgnoreStdlib,
                                  isDebugCompilation,
                                  Invocation.getTargetTriple(),
-                                 *Instance.getDependencyTracker());
+                                 *Instance.getDependencyTracker(),
+                                 Invocation.getIRGenOptions().FilePrefixMap);
   }
 }