Make -cl-kernel-arg-info a cc1-only option like the others

The driver wasn't doing anything with it. Also rephrase the help text.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@199458 91177308-0d34-0410-b5e6-96231b3b80d8
Kill off obsolete err_target_unknown_cxxabi diag

Unused since r199250.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@199459 91177308-0d34-0410-b5e6-96231b3b80d8