pgsql: Use standard C23 and C++ attributes if available

Use standard C23 and C++ attributes if available

Use the standard C23 and C++ attributes [[nodiscard]], [[noreturn]],
and [[maybe_unused]], if available.

This makes pg_nodiscard and pg_attribute_unused() available in
not-GCC-compatible compilers that support C23 as well as in C++.

For pg_noreturn, we can now drop the GCC-specific and MSVC-specific
fallbacks, because the C11 and the C++ implementation will now cover
all required cases.

Note, in a few places, we need to change the position of the attribute
because it's not valid in that place in C23.

Discussion: https://www.postgresql.org/message-id/flat/pxr5b3z7jmkpenssra5zroxi7qzzp6eswuggokw64axmdixpnk(at)zbwxuq7gbbcw

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/76f4b92bac87fa54bd6dd8bd53e59f93127ec2ef

Modified Files
--------------
src/backend/utils/mmgr/slab.c | 2 +-
src/include/c.h | 38 ++++++++++++++++++--------------
src/include/lib/radixtree.h | 2 +-
src/test/modules/worker_spi/worker_spi.c | 2 +-
4 files changed, 25 insertions(+), 19 deletions(-)