RequirementMachine: Remove bogus assertion

We can have a layout requirement 'T : _UnknownLayout', and
_UnknownLayout answers false to isKnownLayout().

Author: slavapestov

include/swift/AST/RewriteSystem.h

@@ -95,7 +95,6 @@ class Atom final {
 
   /// Creates a layout atom, representing a layout constraint.
   static Atom forLayout(LayoutConstraint layout) {
-    assert(layout->isKnownLayout());
     return Atom({}, layout);
   }