Make -save-temps behave like in GCC 4.5.

The -save-temps option now behaves like described in GCC 4.5 release notes
(you can specify output directory for temporary files with -save-temps=obj
-o $DIRNAME). I do not have GCC 4.5 installed, so if there are any
inconsistencies between llvmc and GCC in the implementation of this
feature, please let me know.

llvm-svn: 74190