libstdc++: Improve Doxygen documentation groups [PR 101258]

libstdc++: Improve Doxygen documentation groups [PR 101258]

This defines some new Doxygen groups for C++17 variable templates and
for the contents of <experimental/type_traits>. By documenting the group
as a whole and adding each template to a group we don't need to document
them individually.

Also mark more internals with "@cond undocumented" so that Doxygen
ignores them by default. Also make Doxygen process <experimental/simd>.

For some reason, many of the class templates in <type_traits> do not
appear in the "Metaprogramming" group. For example, add_cv,
remove_extent, and all the is_xxx_constructible and is_xxx_assignable
traits. For some reason, Doxygen doesn't include them in the group,
despite doing it correctly for other traits in the same header.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/101258
	* doc/doxygen/user.cfg.in (INPUT): Add <experimental/simd>.
	(COLS_IN_ALPHA_INDEX): Remove obsolete tag.
	(PREDEFINED): Add/fix some more macros that need to be expanded.
	* include/bits/random.h: Stop Doxygen from documenting internal
	implementation details.
	* include/bits/random.tcc: Likewise.
	* include/bits/this_thread_sleep.h: Fix @file name.
	* include/experimental/bits/simd.h: Add to Doxygen group. Do not
	document internal implementation details.
	* include/experimental/bits/simd_detail.h: Do not document
	internal implementation details.
	* include/experimental/simd: Define Doxygen groups.
	* include/experimental/type_traits: Improve documentation for
	the header file. Define groups. Use @since commands.
	* include/std/scoped_allocator (scoped_allocator_adaptor): Move
	declaration before undocumented region.
	* include/std/type_traits (true_type, false_type): Use using
	declaration instead of typedef.
	(is_invocable_v, is_nothrow_invocable_v, is_invocable_r_v)
	(is_nothrow_invocable_r_v): Move definitions next to other C++17
	variable templates.
	Do not document internal implementation details. Move misplaced
	group-end command. Define group for variable templates.
	* include/std/variant: Do not document internal implementation
	details.
	* testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
	line number.