[libc++] Mark a failing test.

[Mordante]

Summary

The tests fails in debug mode since it manipulates an iterator to a
std::string returned from the dylib. This is a known issue for the
debug iterators.

Updating the libc++ Docker image to Ubuntu Focal caused some breakage.
This was temporary disabled in D112737. This re-enables one of these
tests.

[• Quuxplusone]

it manipulates an iterator to a std::string returned from the dylib

It would be nice to say in the commit message which dylib function is the culprit, and where in this 600-line test the iterator is being manipulated. (I'm aware of std::to_string(int) as a frequent culprit; I just don't know if that's the dylib function you're talking about in this specific case.)

[Mordante]

Disable the test for gcc-11. It seems GCC has an issue with template argument
deduction. The code affected follows the wording of the Standard. This will be
investigated later.
Add more information regarding the other disabled test.

[Mordante]

In D113139#3107289, @Quuxplusone wrote:

it manipulates an iterator to a std::string returned from the dylib

It would be nice to say in the commit message which dylib function is the culprit, and where in this 600-line test the iterator is being manipulated. (I'm aware of std::to_string(int) as a frequent culprit; I just don't know if that's the dylib function you're talking about in this specific case.)

It's numpunct's grouping and the issue is in <format> not in the test itself. I've added this information in the test.

libcxx/test/std/utilities/format/format.functions/locale-specific_form.pass.cpp

 //===----------------------------------------------------------------------===//
 // 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: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-localization
 // UNSUPPORTED: libcpp-has-no-incomplete-format
 
-// TODO(mordante): Investigate these localization/format failures since updating the Docker image in CI
-// UNSUPPORTED: stdlib=libc++
+// The issue is caused in __format_spec::__determine_grouping().
+// There a string iterator is modified. The string is returned
+// from the dylib's use_facet<numpunct<_CharT>>::grouping()
+// XFAIL: LIBCXX-DEBUG-FIXME
+
+// TODO FMT Evaluate gcc-11 status
+// UNSUPPORTED: gcc-11
 
 // REQUIRES: locale.en_US.UTF-8
 
 // <format>
 
 // This test the locale-specific form for these formatting functions:
 //
 //  // [format.functions], formatting functions