Add whole-module optimization option to Swift compiler

Add -whole-module-optimization option as synonym of
-force-single-frontend-invocation (for now). Add support for
-output-file-map when using -whole-module-optimization with multiple
input files -- the key for the single output file's map is the empty string.

<rdar://problem/18603795>

Swift SVN r23625

include/swift/Driver/OutputFileMap.h

@@ -52,6 +52,9 @@ public:
   /// OutputFileMap. (If not present, returns nullptr.)
   const TypeToPathMap *getOutputMapForInput(StringRef Input) const;
 
+  /// Get the map of outputs for a single compile product.
+  const TypeToPathMap *getOutputMapForSingleOutput() const;
+
   /// Dump the OutputFileMap to the given \p os.
   void dump(llvm::raw_ostream &os, bool Sort = false) const;