Make -save-temps behave like in GCC 4.5.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74190 91177308-0d34-0410-b5e6-96231b3b80d8