Introduce TypeBase::isTypeVariableOrMember(). NFC

Similar to “isTypeParameter,” this new entry point determines whether the type is a type variable or a nested type of a type thereof. The latter case isn’t actually formed yet, so this is NFC staging the trivial bits of this change.

lib/AST/Module.cpp

@@ -630,7 +630,8 @@ TypeBase::gatherAllSubstitutions(Module *module,
 
     // If the type is a type variable or is dependent, just fill in empty
     // conformances.
-    if (replacement->is<TypeVariableType>() || replacement->isTypeParameter())
+    if (replacement->isTypeVariableOrMember() ||
+        replacement->isTypeParameter())
       return ProtocolConformanceRef(proto);
 
     // Otherwise, find the conformance.
@@ -743,6 +744,11 @@ Module::lookupConformance(Type type, ProtocolDecl *protocol,
     return None;
   }
 
+  // Type variables have trivial conformances.
+  if (type->isTypeVariableOrMember()) {
+    return ProtocolConformanceRef(protocol);
+  }
+
   // UnresolvedType is a placeholder for an unknown type used when generating
   // diagnostics.  We consider it to conform to all protocols, since the
   // intended type might have.