gf: optimize jl_matching_methods for dispatch tuples (#34745)
When the input type is a dispatch tuple, we know that a subtype lookup
is sufficient, so we can do a slightly more direct lookup and avoid
considering `Union{}`-declared methods as possible results.
This also fixes an issue with the special `jl_matching_methods` query
being done by `jl_insert_backedges`: we would set the `done` flag when a
method "fully covered" the result, even if that method was deleted.
Rather than continue to abuse the meaning of `world == 0` (which also
has performance implications), we add a specific flag for this case.
