[AST] Narrow TypeSubstitutionMap to SubstitutableType keys.

Type substitution works on a fairly narrow set of types: generic type
parameters (to, e.g., use a generic) and archetypes (to map out of a
generic context). Historically, it was also used with
DependentMemberTypes, but recent refactoring to eliminate witness
markers eliminate that code path.

Therefore, narrow TypeSubstitutionMap's keys to SubstitutableType,
which covers archetypes and generic type parameters. NFC

lib/AST/GenericEnvironment.cpp

@@ -79,7 +79,8 @@ Type GenericEnvironment::mapTypeIntoContext(ModuleDecl *M, Type type) const {
 
 Type GenericEnvironment::mapTypeIntoContext(GenericTypeParamType *type) const {
   auto canTy = type->getCanonicalType();
-  auto found = InterfaceToArchetypeMap.find(canTy.getPointer());
+  auto found =
+    InterfaceToArchetypeMap.find(canTy->castTo<GenericTypeParamType>());
   assert(found != InterfaceToArchetypeMap.end() &&
          "missing generic parameter");
   return found->second;