swift234

AST: Refactor GenericParamList::getForwardingSubstitutions() to use GenericSignature::getSubstitutions(), NFC

This is the first, and most trivial, usage of the new
GenericSignature::getSubstitutions() method.

Note that getForwardingSubstitutions() now takes a
GenericSignature, which is slightly awkward.

However, this is in line with our goal of 'hollowing out'
GenericParamList by removing knowledge of the finalized
generic requirements.

Also, there is now a new getForwardingSubstitutionMap()
function, which returns an interface type substitution
mapping. This is used in the new getForwardingSubstitutions()
implementation, and all also be used elsewhere later.

Finally, in the SILFunction we now cache the forwarding
substitutions, instead of re-computing them every time.
I doubt this makes a big difference in performance, but
it's a simple enhancement and every little bit helps.