Improve error message for an unknown --plugin-opt.

Before:

  $ ld.lld --plugin-opt=-foo
  ld.lld: --Unknown command line argument '-abc'

After:

  $ ld.lld --plugin-opt=-foo
  ld.lld: --plugin-opt: ld.lld --Unknown command line argument '-abc'

Differential Revision: https://reviews.llvm.org/D45075

llvm-svn: 328880