[committed] libstdc++: Add missing 'constexpr' to std::tuple [PR102270]

[Jonathan Wakely]

On 16/09/21 23:07 +0100, Jonathan Wakely wrote:
>Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
>
>libstdc++-v3/ChangeLog:
>
>	PR libstdc++/102270
>	* include/std/tuple (_Head_base, _Tuple_impl): Add
>	_GLIBCXX20_CONSTEXPR to allocator-extended constructors.
>	(tuple<>::swap(tuple&)): Add _GLIBCXX20_CONSTEXPR.
>	* testsuite/20_util/tuple/cons/102270.C: New test.

Oops, this test has a .C extension, and so doesn't actually run (and
if I rename it to 102270.cc it fails).

Fix incoming ...