[GSB] Map invalid subject types in requirement constraints to ErrorType

Resolves SR-12072 and rdar://58941114

Author: CodaFi

lib/AST/GenericSignatureBuilder.cpp

@@ -4361,6 +4361,8 @@ ConstraintResult GenericSignatureBuilder::addTypeRequirement(
 
       if (subjectType->is<DependentMemberType>()) {
         subjectType = resolveDependentMemberTypes(*this, subjectType);
+      } else {
+        subjectType = ErrorType::get(subjectType);
       }
 
       auto invalidConstraint = Constraint<Type>(

validation-test/compiler_crashers_2_fixed/rdar58941114.swift

@@ -0,0 +1,6 @@
+// RUN: not %target-swift-frontend %s -typecheck
+
+class C {}
+protocol Foo {
+  associatedtype X where C: X
+}