[libc++] Use -fexperimental-library when available to enable experimental features

[ldionne]

LGTM if CI passes. Thanks!

libcxx/utils/libcxx/test/params.py

183–184

This can go away now.

[philnik]

Address comments

[philnik]

Rebased

[philnik]

Try to fix CI

libcxx/utils/libcxx/test/params.py

 # ===----------------------------------------------------------------------===##
 #
 # 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
 #
 # ===----------------------------------------------------------------------===##
 import sys
 import re
 import shlex
 from pathlib import Path
 
 from libcxx.test.dsl import *
-from libcxx.test.features import _isMSVC
+from libcxx.test.features import _isGCC
 
 
 _warningFlags = [
     "-Werror",
     "-Wall",
     "-Wctad-maybe-unsupported",
     "-Wextra",
     "-Wshadow",
Parameter(
 
                  The Standard libraries currently supported are:
                  - llvm-libc++: The 'upstream' libc++ as shipped with LLVM.
                  - apple-libc++: libc++ as shipped by Apple. This is basically like the LLVM one, but
                                  there are a few differences like installation paths, the use of
                                  universal dylibs and the existence of availability markup.
                  - libstdc++: The GNU C++ library typically shipped with GCC.
                  - msvc: The Microsoft implementation of the C++ Standard Library.
                 """,
         actions=lambda stdlib: filter(
             None,
             [
                 AddFeature("stdlib={}".format(stdlib)),
                 # Also add an umbrella feature 'stdlib=libc++' for all flavors of libc++, to simplify
                 # the test suite.
                 AddFeature("stdlib=libc++") if re.match(".+-libc\+\+", stdlib) else None,
             ],
         ),
DEFAULT_PARAMETERS = [
     ),
     Parameter(
         name="enable_experimental",
         choices=[True, False],
         type=bool,
         default=True,
         help="Whether to enable tests for experimental C++ Library features.",
         actions=lambda experimental: [
-            # When linking in MSVC mode via the Clang driver, a -l<foo>
-            # maps to <foo>.lib, so we need to use -llibc++experimental here
-            # to make it link against the static libc++experimental.lib.
-            # We can't check for the feature 'msvc' in available_features
-            # as those features are added after processing parameters.
             AddFeature("c++experimental"),
-            PrependLinkFlag(lambda cfg: "-llibc++experimental" if _isMSVC(cfg) else "-lc++experimental"),
-            AddCompileFlag("-D_LIBCPP_ENABLE_EXPERIMENTAL"),
+            PrependLinkFlag(lambda cfg: "-lc++experimental" if _isGCC(cfg) else "-fexperimental-library"),
+            AddCompileFlag(lambda cfg: "-D_LIBCPP_ENABLE_EXPERIMENTAL" if _isGCC(cfg) else "-fexperimental-library"),
         ]
         if experimental
         else [
             AddFeature("libcpp-has-no-incomplete-pstl"),
             AddFeature("libcpp-has-no-experimental-stop_token"),
             AddFeature("libcpp-has-no-incomplete-tzdb"),
             AddFeature("libcpp-has-no-experimental-syncstream"),
         ],