Revert "Change -save-temps to emit unoptimized bitcode files."

This reverts commit r224503.

It broke compilation of fortran through the Clang driver. Previously
`clang -c t.f` would invoke `gcc t.f` and `clang -cc1as`, but now it
tries to call `clang -cc1 t.f` which fails for obvious reasons.

llvm-svn: 224546