[libc++abi] Add a test to ensure the abi namespace alias is declared correctly (llvm#167485)

Author: philnik777

libcxxabi/test/namespace.compile.pass.cpp

@@ -0,0 +1,15 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <cxxabi.h>
+
+// Make sure the `abi` namespace already exists
+namespace abi_should_exist = abi;
+
+// Make sure `abi` is an alias for `__cxxabiv1`
+namespace abi = __cxxabiv1;