Invoke the Darwin assembler with -g instead of --gdwarf2.

Invoke the Darwin assembler with -g instead of --gdwarf2.

The -g and --gdwarf2 options are currently synonyms to the Darwin assembler.
But clang itself does not recognize --gdwarf2, so if we want to experiment
with using clang, with its integrated assembler, to replace the default
assembler, it is necessary to use -g.  <rdar://problem/10349486>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143533 91177308-0d34-0410-b5e6-96231b3b80d8