Reapply a subset of r167567 to clean up Darwin-specific code for invoking gcc.

Reapply a subset of r167567 to clean up Darwin-specific code for invoking gcc.

Unlike my previous attempt at this, this patch leaves intact the check for
whether clang can handle the input file type, and for non-Darwin toolchains it
will invoke gcc for things it cannot handle. For Darwin toolchains, the
behavior reported in pr14338 still occurs with this patch, but that is a
definite improvement from what happens currently, where it just crashes with
an assertion failure.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168505 91177308-0d34-0410-b5e6-96231b3b80d8