swift234

AST: Add GenericSignature::getSubstitutions()

This is the opposite of GenericSignature::getSubstitutionMap(),
transforming an interface type substitution mapping into
a Substitution array.

Previously, Substitution arrays were built with hand-rolled
logic, usually relying on a GenericParamList's AllArchetypes
list. We want to stop using the AllArchetypes list, instead
using the requirements array from a GenericSignature.

Nothing calls this method yet; existing code will be refactored
to call it in the next few patches.