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/CSGen.cpp

@@ -3165,6 +3165,9 @@ bool swift::isExtensionApplied(DeclContext &DC, Type BaseTy,
       case RequirementKind::Conformance:
         createMemberConstraint(Req, ConstraintKind::ConformsTo);
         break;
+      case RequirementKind::Layout:
+        createMemberConstraint(Req, ConstraintKind::Layout);
+        break;
       case RequirementKind::Superclass:
         createMemberConstraint(Req, ConstraintKind::Subtype);
         break;