swift234

SIL: Use interface types in the SIL parser

The SIL parser used PolymorphicFunctionType in two places:

- Internals of SILFunctionType parsing

- Overload selection for class_method / super_method / dynamic_method
  instructions

It is better to have Sema construct GenericFunctionType directly
in SIL mode. In particular, the overload selection logic is simpler
now, since it does not have to deal with the fact that
PolymorphicFunctionTypes do not canonicalize.

Mostly NFC, except the SIL printer output is a bit different; for a
generic method on a generic type, the type parameters all come first,
like ``<T><U> G<T> -> (U) -> ()'' -vs- ``<T> G<T> -> <U> (U) -> ()''.

Also, generic constraints look different, instead of ``<`Self` : P>``
we now have ``<Self where Self : P>''.

This patch has two consequences that will become important later:

- While code that constructs PolymorphicFunctionType still exists in
  Sema, the SIL parser was the last major component that *consumed*
  PolymorphicFunctionType.

- Everywhere we set SILFunction::ContextGenericParams, we now have
  a well-formed context GenericSignature available, allowing
  ContextGenericParams to be replaced by a GenericSignature
  eventually.