[ELF] Refactor the way we handle -plugin-opt= (GCC collect2 or clang LTO related options)

GCC collect2 passes several options to the linker even if LTO is not used
(note, lld does not support GCC LTO). The lto-wrapper may be a relative
path (especially during development, when gcc is in a build directory), e.g.

  -plugin-opt=relative/path/to/lto-wrapper

We need to ignore such options, which are currently interpreted by
cl::ParseCommandLineOptions() and will fail with `error: --plugin-opt: ld.lld: Unknown command line argument 'relative/path/to/lto-wrapper'`
because the path is apparently not an option registered by an `llvm::cl::opt`.

See lto-plugin-ignore.s for how we interpret various -plugin-opt= options now.

Reviewed By: grimar, tejohnson

Differential Revision: https://reviews.llvm.org/D78158