swift234

AST: Refactor GenericParamList::getSubstitutionMap() to take a GenericSignature, NFC

This function takes a substitution array and produces a
contextual type substitution map, so it is the contextual
type equivalent of GenericSignature::getSubstitutionMap(),
which produces an interface type substitution map.

The new version takes a GenericSignature, just like the new
getForwardingSubstitutions(), so that it can walk the
requirements of the signature rather than walking the
AllArchetypes list.

Also, this new version now produces a mapping from
archetypes to conformances in addition to the type mapping,
which will allow it to be used in a few places that had
hand-coded logic.