[NFC] Correct constraint system assumption

We currently assume that, if a subscript is declared within a value type’s decl, it must need `self` to be passed inout. This isn’t true for static subscripts, because even though the DeclContext is a value type, the metatype is actually a reference type. Skip this check for non-instance members.

NFC until static subscripts are added.

lib/Sema/ConstraintSystem.cpp

@@ -1287,7 +1287,8 @@ ConstraintSystem::getTypeOfMemberReference(
     // If self is a value type and the base type is an lvalue, wrap it in an
     // inout type.
     auto selfFlags = ParameterTypeFlags();
-    if (!outerDC->getDeclaredInterfaceType()->hasReferenceSemantics() &&
+    if (isInstance &&
+        !outerDC->getDeclaredInterfaceType()->hasReferenceSemantics() &&
         baseTy->is<LValueType>() &&
         !selfTy->hasError())
       selfFlags = selfFlags.withInOut(true);