Add a new spelling for module map files 'module.modulemap'

This name, while more verbose, plays more nicely with tools that use
file extensions to determine file types. The existing spelling
'module.map' will continue to work, but the new spelling will take
precedence.

In frameworks, this new filename will only go in a new 'Modules'
sub-directory.

Similarly, add a module.private.modulemap corresponding to
module_private.map.

llvm-svn: 204261

clang/test/Modules/Inputs/ModuleMapLocations/Module_ModuleMap/a.h

@@ -0,0 +1 @@
+void will_be_found1(void);

clang/test/Modules/Inputs/ModuleMapLocations/Module_ModuleMap/module.modulemap

@@ -0,0 +1,3 @@
+module module_modulemap {
+  header "a.h"
+}