Implement P2128R6 in C++23 mode.
Unlike GCC's implementation, this doesn't try
to recover when a user meant to use a comma expression.
Because the syntax changes meaning in C++23, the patch is
*NOT* implemented as an extension. Instead, declaring an array
with not exactly 1 parameter is an error in older languages
modes. There is an off-by-default extension warning in C++23 mode.
Unlike the standard, we supports default arguments;
Ie, we assume, based on conversations in WG21, that
the proposed resolution to CWG2507 will be accepted.
We allow arrays OpenMP sections and C++23 multidimensional array
[a , b] multi dimensional array
[a : b] open mp section
[a, b: c] // error
The rest of the patch is relatively straight forward: we
take care to support arbitrary number of argument everywhere.