Fix logic to detect cl::option equality. (#65754)

This is a new attempt of https://reviews.llvm.org/D159481, this time as
GitHub PR.

`GenericOptionValue::compare()` should return `true` for a match.

- `OptionValueBase::compare()` always returns `false` and shouldn't
match anything.
- `OptionValueCopy::compare()` returns `false` if not `Valid` which
corresponds to no match.

Also adding some tests.