[test] Add crasher test for Clang type serialization.

Author: varungandhi-apple

test/Serialization/Inputs/non-modular-header.h

@@ -0,0 +1,3 @@
+typedef struct {
+  int x;
+} PlaceholderType;

test/Serialization/non-modular-clang-type.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+// RUN: not --crash %target-swift-frontend(mock-sdk: %clang-importer-sdk) %s -enable-testing -module-name NonModularApp -emit-module -o %t/NonModularApp.swiftmodule -import-objc-header %S/Inputs/non-modular-header.h -DNON_MODULAR_APP -use-clang-function-types 2>&1 | %FileCheck %s
+
+// CHECK: Clang function type is not serializable
+
+#if NON_MODULAR_APP
+import ctypes
+struct S {
+  static func f(_ : @convention(c, cType: "void (*)(PlaceholderType, size_t)") (PlaceholderType, Int) -> ()) {}
+}
+#endif