testsuite: Add plugin to verify bits/std.cc exports

testsuite: Add plugin to verify bits/std.cc exports

The following patch adds another g++.dg/plugin/ testsuite plugin,
this time to verify whether some std.cc exports aren't mistakenly
omitted.

The patch is a reworked version of the
https://gcc.gnu.org/pipermail/libstdc++/2025-August/thread.html#62859
proof of concept.  That version just dumped out everything it saw
in the std namespace and its child namespaces (excluding non-inline
subnamespaces with identifiers starting with underscore) and then I've
used sed&grep to form a list of omissions.

This patch keeps the previous walk of std namespace and namespaces children
of it, but it only reports (in this version using error_at instead of inform
previously) what it finds if it isn't exported from the module and is not
deprecated (deprecated attribute is used usually for zombie.names in the
standard).

I've been strugling with the detection of what is and what isn't exported,
had to try several different methods.
What is DECL_MODULE_EXPORT_P is ignored, but that is not set on everything
actually exported.  In other cases there is OVL_EXPORT_P flag on OVERLOAD
(but OVL_HIDDEN_P at the start doesn't have it).  Another case are inline
namespaces, e.g. for std::filesystem::__cxx11::begin or
std::filesystem::__cxx11::directory_iterator.  In the latter case, there
is no sign of the above flags in __cxx11 binding entry, but there is a
USING_DECL with the same name directly in std::filesystem.  And for begin
there is OVERLOAD with OVL_EXPORT_P in std::filesystem but not in
std::filesystem::__cxx11.

2026-04-29  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/plugin/plugin.exp: Set PLUGIN_DEFAULT_REPO.  Add
	set*module*exports* to plugin_test_list.  Remove *.gcm files
	at the start and end.
	* g++.dg/plugin/std_module_exports_plugin.cc: New file.
	* g++.dg/plugin/std-module-exports-c++20.C: New test.
	* g++.dg/plugin/std-module-exports-c++23.C: New test.
	* g++.dg/plugin/std-module-exports-c++26.C: New test.

Reviewed-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>