[libc++] Move libcxx/test/libcxx/extensions to libcxx/test/extensions and update the tests

This patch adds a separate `extensions` directory, since there are
quite a few extensions in libc++ that aren't necessarily
libc++-specific. For example, the tests currently in
`libcxx/test/libcxx/extensions` should also pass with libstdc++, since
they originally added the extension. This also "documents" what users
are allowed to rely on and what parts are just libc++ tests to make sure
our implementation is behaving as we expect, which may be subject to
change.

Author: philnik777

libcxx/test/extensions/gnu/hash/specializations.pass.cpp

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Prevent <ext/hash_set> from generating deprecated warnings for this test.
+// ADDITIONAL_COMPILE_FLAGS: -Wno-deprecated
+
+#include <cassert>
+#include <ext/hash_map>
+#include <string>
+
+int main(int, char**) {
+  char str[] = "test";
+  assert(__gnu_cxx::hash<const char*>()("test") == std::hash<std::string>()("test"));
+  assert(__gnu_cxx::hash<char*>()(str) == std::hash<std::string>()("test"));
+  assert(__gnu_cxx::hash<char>()(42) == 42);
+  assert(__gnu_cxx::hash<signed char>()(42) == 42);
+  assert(__gnu_cxx::hash<unsigned char>()(42) == 42);
+  assert(__gnu_cxx::hash<short>()(42) == 42);
+  assert(__gnu_cxx::hash<unsigned short>()(42) == 42);
+  assert(__gnu_cxx::hash<int>()(42) == 42);
+  assert(__gnu_cxx::hash<unsigned int>()(42) == 42);
+  assert(__gnu_cxx::hash<long>()(42) == 42);
+  assert(__gnu_cxx::hash<unsigned long>()(42) == 42);
+
+  return 0;
+}

libcxx/test/libcxx/extensions/hash/specializations.compile.fail.cpp renamed to libcxx/test/extensions/gnu/hash/specializations.verify.cpp

@@ -6,13 +6,14 @@
 //
 //===----------------------------------------------------------------------===//
 
+// ADDITIONAL_COMPILE_FLAGS: -Wno-deprecated
+
 #include <assert.h>
 #include <ext/hash_map>
 #include <string>
 
-int main(int, char**)
-{
-    assert(__gnu_cxx::hash<std::string>()(std::string()) == 0);  // error
+int main(int, char**) {
+  assert(__gnu_cxx::hash<std::string>()(std::string()) == 0); // expected-error {{does not provide a call operator}}
 
   return 0;
 }

libcxx/test/libcxx/extensions/hash_map/const_iterator.compile.fail.cpp renamed to libcxx/test/extensions/gnu/hash_map/const_iterator.verify.cpp

@@ -6,14 +6,15 @@
 //
 //===----------------------------------------------------------------------===//
 
+// ADDITIONAL_COMPILE_FLAGS: -Wno-deprecated
+
 #include <ext/hash_map>
 
-int main(int, char**)
-{
-    __gnu_cxx::hash_map<int, int> m;
-    m[1] = 1;
-    const __gnu_cxx::hash_map<int, int> &cm = m;
-    cm.find(1)->second = 2;  // error
+int main(int, char**) {
+  __gnu_cxx::hash_map<int, int> m;
+  m[1]                                    = 1;
+  const __gnu_cxx::hash_map<int, int>& cm = m;
+  cm.find(1)->second = 2; // expected-error {{cannot assign to return value because function 'operator->' returns a const value}}
 
   return 0;
 }

libcxx/test/extensions/gnu/lit.local.cfg

@@ -0,0 +1,5 @@
+
+# Only libc++ and libstdc++ have any gnu extensions
+if ("stdlib=libc++" not in config.available_features and
+    "stdlib=libstdc++" not in config.available_features):
+    config.unsupported = True