Add support for layout requirements with layout constraints.

This commit introduces new kind of requirements: layout requirements.

This kind of requirements allows to expose that a type should satisfy certain layout properties, e.g. it should be a trivial type, have a given size and alignment, etc.

lib/Sema/ConstraintSystem.cpp

@@ -962,6 +962,12 @@ void ConstraintSystem::openGeneric(
       break;
     }
 
+    case RequirementKind::Layout: {
+      // Do not process layout constraints yet, until we allow their use
+      // outside of @_specialize attribute.
+      break;
+    }
+
     case RequirementKind::Superclass: {
       auto subjectTy = req.getFirstType().transform(replaceDependentTypes);
       auto boundTy = req.getSecondType().transform(replaceDependentTypes);