[Driver] Refactor err_drv_unsupported_option_argument call sites to use llvm::opt::Arg::getSpelling

For `-foo=bar`, getSpelling return `-foo=` which is exactly what we need from
the diagnostic. Drop `-` from the err_drv_unsupported_option_argument template.
This change makes `--` long option diagnostics more convenient.

Reviewed By: lenary

Differential Revision: https://reviews.llvm.org/D137659