Fix '-main-file-name <name>' so that it is used for the ModuleID.

Summary:
Previously, you could not specify the original file name when passing a preprocessed file into the compiler
Now you can use 'clang -Xclang -main-file-name -Xclang <original file name> ...'
Or 'clang -cc1 -main-file-name <original file name> ...'