AST: Rename 'canonical wrt. generic signature' to 'reduced'

We had two notions of canonical types, one is the structural property
where it doesn't contain sugared types, the other one where it does
not contain reducible type parameters with respect to a generic
signature.

Rename the second one to a 'reduced type'.

lib/Sema/ConstraintSystem.cpp

@@ -1988,7 +1988,7 @@ typeEraseExistentialSelfReferences(
 
       // If the type parameter is beyond the domain of the existential generic
       // signature, ignore it.
-      if (!existentialSig->isValidTypeInContext(t)) {
+      if (!existentialSig->isValidTypeParameter(t)) {
         return Type(t);
       }
 
@@ -6540,7 +6540,7 @@ static bool doesMemberHaveUnfulfillableConstraintsWithExistentialBase(
         return Action::SkipChildren;
       }
 
-      if (!Sig->isValidTypeInContext(ty)) {
+      if (!Sig->isValidTypeParameter(ty)) {
         return Action::SkipChildren;
       }