Make debug info work when using -save-temps.

Make debug info work when using -save-temps.
 - This is pretty ugly, but the most obvious solution. Chime in if you
   have a nicer one.

 - The problem is that with -save-temps, clang-cc has no idea what the
   name of the original input file is. However, the user expects to be
   able to set breakpoints based on the input file name.

 - We support this by providing a new option -main-file-name (similar
   to -dumpbase used by gcc) which allows the driver to pass in the
   original file name.

 - <rdar://problem/6753383> building with clang using --save-temps
   gets the compile unit name from the .i file...

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@68595 91177308-0d34-0410-b5e6-96231b3b80d8