swift234

SIL: Add devirtualizer support for default witness methods

We ignore substitutions from the conformance, using the Self type
substitution from the call site instead.

The new SILFunctionType::getDefaultWitnessMethodProtocol() method
is used to figure out what "shape" the Self substitutions need
to take.

This is cleaner than it was before the method was added, but is
still a bit of a hack; more and more it appears that we need to
stop thinking of witness_method as a separate calling convention,
and design what @rjmccall described as "abstraction patterns for
generic signatures" instead.