[libc++] Enable tests for the experimental library by default

[ldionne]

Summary

This matches the fact that we build the experimental library by default.
Otherwise, by default we'd be building the library but not testing it,
which is inconsistent.

[ldionne]

Try to fix libc++abi's and libunwind's builds

[ldionne]

Fix tests against the static library with libc++experimental enabled.

[ldionne]

Tests for the static library were fixed and tested locally, shipping now.

[ldionne]

Rebase onto main.

[ldionne]

Only failure is the known Runtimes build failure that we're fixing in D103533 -- shipping.

libcxx/utils/libcxx/test/params.py

Parameter(name='use_sanitizer', choices=['', 'Address', 'Undefined', 'Memory', 'MemoryWithOrigins', 'Thread', 'DataFlow', 'Leaks'], type=str, default='',
 
               AddFlag('-fsanitize=dataflow') if sanitizer == 'DataFlow' else None,
               AddFlag('-fsanitize=leaks') if sanitizer == 'Leaks' else None,
 
               AddFeature('sanitizer-new-delete') if sanitizer in ['Address', 'Memory', 'MemoryWithOrigins', 'Thread'] else None,
             ])),
 
   # Parameters to enable or disable parts of the test suite
-  Parameter(name='enable_experimental', choices=[True, False], type=bool, default=False,
+  Parameter(name='enable_experimental', choices=[True, False], type=bool, default=True,
             help="Whether to enable tests for experimental C++ libraries (typically Library Fundamentals TSes).",
             actions=lambda experimental: [] if not experimental else [
               AddFeature('c++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.

libcxxabi/test/lit.site.cfg.in

 config.enable_shared            = @LIBCXXABI_LINK_TESTS_WITH_SHARED_LIBCXX@
 config.enable_exceptions        = @LIBCXXABI_ENABLE_EXCEPTIONS@
 config.host_triple              = "@LLVM_HOST_TRIPLE@"
 if "@TARGET_TRIPLE@":
     config.target_triple        = "@TARGET_TRIPLE@"
 config.sysroot                  = "@LIBCXXABI_SYSROOT@"
 config.gcc_toolchain            = "@LIBCXXABI_GCC_TOOLCHAIN@"
 config.cxx_ext_threads          = @LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY@
+config.enable_experimental      = False
 
 config.pstl_src_root            = "@ParallelSTL_SOURCE_DIR@" if @LIBCXX_ENABLE_PARALLEL_ALGORITHMS@ else None
 config.pstl_obj_root            = "@ParallelSTL_BINARY_DIR@" if @LIBCXX_ENABLE_PARALLEL_ALGORITHMS@ else None
 
 # Code signing
 config.llvm_codesign_identity   = "@LLVM_CODESIGNING_IDENTITY@"
 
 site.addsitedir(os.path.join(config.libcxxabi_src_root, 'test'))

libunwind/test/lit.site.cfg.in

 config.enable_shared            = @LIBCXX_ENABLE_SHARED@
 config.arm_ehabi                = @LIBUNWIND_USES_ARM_EHABI@
 config.host_triple              = "@LLVM_HOST_TRIPLE@"
 if "@TARGET_TRIPLE@":
     config.target_triple        = "@TARGET_TRIPLE@"
 config.sysroot                  = "@LIBUNWIND_SYSROOT@"
 config.gcc_toolchain            = "@LIBUNWIND_GCC_TOOLCHAIN@"
 config.cxx_ext_threads          = @LIBUNWIND_BUILD_EXTERNAL_THREAD_LIBRARY@
+config.enable_experimental      = False
 
 site.addsitedir(os.path.join(config.libunwind_src_root, 'test'))
 site.addsitedir(os.path.join(config.libcxx_src_root, 'utils'))
 
 # name: The name of this test suite.
 config.name = 'libunwind'
 
 # suffixes: A list of file extensions to treat as test files.