[Refactoring] Skip failed witness decls in "add codable implementation"

When there are problems in the properties in the target type, witness
methods may not be synthesized. Don't try to add such methods.

#72387

Author: rintaro

lib/Refactoring/AddExplicitCodableImplementation.cpp

@@ -133,7 +133,7 @@ class AddCodableContext {
           kind == KnownProtocolKind::Decodable) {
         for (auto requirement : protocol->getProtocolRequirements()) {
           auto witness = conformance->getWitnessDecl(requirement);
-          if (witness->isSynthesized()) {
+          if (witness && witness->isSynthesized()) {
             Printer.printNewline();
             witness->print(Printer, Options);
             Printer.printNewline();

test/refactoring/AddCodableImplementation/Outputs/has_error/invalid_member.swift.expected

@@ -0,0 +1,9 @@
+
+struct Foo: Codable {
+    var other: Unknown
+
+    private enum CodingKeys: CodingKey {
+      case other
+    }
+}
+

test/refactoring/AddCodableImplementation/has_error.swift

@@ -0,0 +1,8 @@
+// RUN: %empty-directory(%t.result)
+
+// RUN: %refactor -add-explicit-codable-implementation -source-filename %s -pos=%(line + 2):8 > %t.result/invalid_member.swift
+// RUN: diff -u %S/Outputs/has_error/invalid_member.swift.expected %t.result/invalid_member.swift
+struct Foo: Codable {
+    var other: Unknown
+}
+