[C2y] Implement WG14 N3369 and N3469 (_Countof) (#133125)
C2y adds the `_Countof` operator which returns the number of elements in
an array. As with `sizeof`, `_Countof` either accepts a parenthesized
type name or an expression. Its operand must be (of) an array type. When
passed a constant-size array operand, the operator is a constant
expression which is valid for use as an integer constant expression.
This is being exposed as an extension in earlier C language modes, but
not in C++. C++ already has `std::extent` and `std::size` to cover these
needs, so the operator doesn't seem to get the user enough benefit to
warrant carrying this as an extension.
Fixes #102836
