Factor a transform visitor out of IsBindableVisitor.

For substituted function type lowering, we want to take every structural position where a
concrete type matches the original generic type in an interface, and replace it with an
independent generic parameter. The first bit, matching up structural positions to generic
parameters, is more or less what `Type::isBindableTo` already does, so we can adapt its
visitor to take a callback that optionally substitutes into the type being walked based
on the generic argument and binding in the given pair of types.

Author: jckarter

include/swift/AST/Types.h

@@ -872,7 +872,19 @@ class alignas(1 << TypeAlignInBits) TypeBase {
   /// True if this type contains archetypes that could be substituted with
   /// concrete types to form the argument type.
   bool isBindableTo(Type ty);
+  
+  /// Visit this type and the argument type in parallel, invoking the callback
+  /// function with each archetype-to-substituted-type binding. The callback
+  /// may return a new type to substitute into the result type, or return
+  /// CanType() to error out of the operation.
+  ///
+  /// Returns the substituted type, or a null CanType() if this type
+  /// is not bindable to the substituted type, or the callback returns
+  /// CanType().
+  CanType substituteBindingsTo(Type ty,
+         llvm::function_ref<CanType(ArchetypeType*, CanType)> substFn);
 
+  
   /// Determines whether this type is similar to \p other as defined by
   /// \p matchOptions.
   bool matches(Type other, TypeMatchOptions matchOptions);
@@ -4274,6 +4286,10 @@ class SILFunctionType final : public TypeBase, public llvm::FoldingSetNode,
   SILType substInterfaceType(SILModule &M,
                              SILType interfaceType) const;
 
+  /// Return the unsubstituted function type equivalent to this type; that is, the type that has the same
+  /// argument and result types as `this` type after substitutions, if any.
+  CanSILFunctionType getUnsubstitutedType(SILModule &M) const;
+                                    
   void Profile(llvm::FoldingSetNodeID &ID) {
     Profile(ID, getSubstGenericSignature(), getExtInfo(), getCoroutineKind(),
             getCalleeConvention(), getParameters(), getYields(), getResults(),