RequirementMachine: Fix assertion when protocol Self is constrained to a non-conforming type

The conflicting rule here is the permanent 'identity conformance'
([P].[P] => [P]). Permanent rules cannot be marked as conflicting,
so just check for this condition first.

Author: slavapestov

lib/AST/RequirementMachine/ConcreteTypeWitness.cpp

@@ -159,7 +159,8 @@ void PropertyMap::concretizeNestedTypesFromConcreteParent(
           concreteRule.markConflicting();
 
         auto &conformanceRule = System.getRule(conformanceRuleID);
-        if (conformanceRule.getRHS().size() == key.size())
+        if (!conformanceRule.isIdentityConformanceRule() &&
+            conformanceRule.getRHS().size() == key.size())
           conformanceRule.markConflicting();
       }
 

test/Generics/protocol_self_concrete_error.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift -requirement-machine-protocol-signatures=on
+
+// This triggered a Requirement Machine assertion because the permanent
+// 'identity conformance' rule ([P].[P] => [P]) was the source of the
+// conflict.
+
+struct S {}
+
+protocol P where Self == S {}