Add a test to cover structural match for recursive data types

This didn't use to work prior to r370639, now that this is supported
add a testcase to prevent regressions.

rdar://problem/53602368

clang/test/Modules/Inputs/rec-types/a.h

@@ -0,0 +1,2 @@
+// rec-types/a.h
+#include "b.h"

clang/test/Modules/Inputs/rec-types/b.h

@@ -0,0 +1,2 @@
+// rec-types/b.h
+#include "c.h"

clang/test/Modules/Inputs/rec-types/c.h

@@ -0,0 +1,7 @@
+struct some_descriptor
+{
+  // commenting line above make this struct work
+  void *(*thunk)(struct some_descriptor *);
+  unsigned long key;
+};
+

clang/test/Modules/Inputs/rec-types/module.modulemap

@@ -0,0 +1,9 @@
+module a {
+  header "a.h"
+  // Hide content by not re-exporting module b.
+}
+
+module b {
+  header "b.h"
+  export *
+}

clang/test/Modules/structural-equivalent-recursive-types.c

@@ -0,0 +1,7 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I%S/Inputs/rec-types -fsyntax-only %s -verify
+#include "a.h"
+#include "c.h"
+void foo(struct some_descriptor *st) { (void)st->thunk; }
+
+// expected-no-diagnostics