[libc++] Mark a failing test.

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.

Reviewed By: ldionne, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D113139

Author: mordante

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

@@ -10,8 +10,13 @@
 // 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