Add frontend flag -import-module <name>.

This implicitly adds the named module as an import of every source file
in the module being compiled. This is not intended to be used generally,
but will be useful for playgrounds.

rdar://problem/19605934

Swift SVN r24905

include/swift/Frontend/FrontendOptions.h

@@ -72,6 +72,9 @@ public:
   /// by the frontend.
   std::string OutputFilename;
 
+  /// An arbitrary module to import and make implicitly visible.
+  std::string ImplicitImportModuleName;
+
   /// An Objective-C header to import and make implicitly visible.
   std::string ImplicitObjCHeaderPath;