base: Deprecate the GEM5_NO_DISCARD macro.
base: Deprecate the GEM5_NO_DISCARD macro.

The now standard [[nodiscard]] attribute can be used directly instead.
Unfortunately, I can't think of any way to actually mark the old macro
as deprecated, since it still has to expand to an attribute which
applies to the following function.

Change-Id: Icbbe3e3d182d845f289727724fef080722093683
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48510
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Maintainer: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>

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