[modules] Add a regression test for merging anon decls in extern C contexts.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316444 91177308-0d34-0410-b5e6-96231b3b80d8

Author: vgvassilev

test/Modules/merge-anon-in-extern_c.cpp

@@ -0,0 +1,19 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fmodules-cache-path=%t -verify %s
+// expected-no-diagnostics
+
+#pragma clang module build sys_types
+module sys_types {}
+#pragma clang module contents
+#pragma clang module begin sys_types
+extern "C" {
+  typedef union { bool b; } pthread_mutex_t;
+}
+#pragma clang module end
+#pragma clang module endbuild
+
+typedef union { bool b; } pthread_mutex_t;
+#pragma clang module import sys_types
+
+const pthread_mutex_t *m;
+