third_party/libcxxabi: Add test suite (#1076)

Added the `libcxxabi` test suite as found in LLVM 17.0.6.

Some tests that do not apply to the current configuration of
comsopolitan are not added. These include:
- `backtrace_test`, `forced_unwind*`: Use unwind function unsupported in
SjLj mode.
- `noexception*`: Designed to test `libcxxabi` in no exceptions mode.

Some tests are added but not enabled due to bugs specific to GCC or
cosmopolitan. These are clearly indicated in the `BUILD.mk` file.

third_party/libcxxabi/test/catch_const_pointer_nullptr.pass.cc

@@ -0,0 +1,148 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: no-exceptions
+
+#include "third_party/libcxx/cassert"
+
+// Clang emits  warnings about exceptions of type 'Child' being caught by
+// an earlier handler of type 'Base'. Congrats clang, you've just
+// diagnosed the behavior under test.
+#if defined(__clang__)
+#pragma clang diagnostic ignored "-Wexceptions"
+#endif
+
+#if __has_feature(cxx_nullptr)
+
+struct A {};
+
+void test1()
+{
+    try
+    {
+        throw nullptr;
+        assert(false);
+    }
+    catch (A* p)
+    {
+        assert(!p);
+    }
+    catch (const A*)
+    {
+        assert(false);
+    }
+}
+
+
+void test2()
+{
+    try
+    {
+        throw nullptr;
+        assert(false);
+    }
+    catch (const A* p)
+    {
+        assert(!p);
+    }
+    catch (A*)
+    {
+        assert(false);
+    }
+}
+
+void test3()
+{
+    try
+    {
+        throw nullptr;
+        assert(false);
+    }
+    catch (const A* const p)
+    {
+        assert(!p);
+    }
+    catch (A*)
+    {
+        assert(false);
+    }
+}
+
+void test4()
+{
+    try
+    {
+        throw nullptr;
+        assert(false);
+    }
+    catch (A* p)
+    {
+        assert(!p);
+    }
+    catch (const A* const)
+    {
+        assert(false);
+    }
+}
+
+void test5()
+{
+    try
+    {
+        throw nullptr;
+        assert(false);
+    }
+    catch (A const* p)
+    {
+        assert(!p);
+    }
+    catch (A*)
+    {
+        assert(false);
+    }
+}
+
+void test6()
+{
+    try
+    {
+        throw nullptr;
+        assert(false);
+    }
+    catch (A* p)
+    {
+        assert(!p);
+    }
+    catch (A const*)
+    {
+        assert(false);
+    }
+}
+
+
+#else
+
+void test1() {}
+void test2() {}
+void test3() {}
+void test4() {}
+void test5() {}
+void test6() {}
+
+#endif
+
+int main(int, char**) {
+    test1();
+    test2();
+    test3();
+    test4();
+    test5();
+    test6();
+
+    return 0;
+}