RequirementMachine: Fix an incorrect assertion

We don't expect to see type parameters that are not generic parameters
here, but dependent member types that wrap an ErrorType are fine, they
show up when a conformance had an invalid type witness.

Fixes the remaining example from #59384.

Author: slavapestov

lib/AST/RequirementMachine/TypeDifference.cpp

@@ -332,16 +332,18 @@ swift::rewriting::buildTypeDifference(
               return nextSubstitution(substitution);
             }
 
-            assert(!t->is<DependentMemberType>());
+            // DependentMemberType with ErrorType base is OK.
+            assert(!t->isTypeParameter());
             return std::nullopt;
           });
         }
       }
 
-      assert(!t->is<DependentMemberType>());
       return nextSubstitution(substitutions[index]);
     }
 
+    // DependentMemberType with ErrorType base is OK.
+    assert(!t->isTypeParameter());
     return std::nullopt;
   });
 

validation-test/compiler_crashers_2_fixed/issue-59384.swift

@@ -0,0 +1,16 @@
+// RUN: not %target-swift-frontend -emit-ir %s
+
+protocol P1 {}
+protocol Q1 {
+  associatedtype A
+  associatedtype B
+}
+
+protocol Q2: Q1 where B == S<Self>, B.C == Self {}
+
+protocol P2: P1 {
+  associatedtype C: Q2 where C.A == Void
+}
+
+struct S<C: Q2>: P2 {
+}