[stmt.expand] Follow up CWG3048

[xmcgcg]

Full name of submitter: Geng Cheng

Reference: [stmt.expand]

Issue description:
The resolution of CWG3048 introduced the N=0 special case for destructuring expansion statements. The expansion result includes the following declaration:

constexpr_opt auto&& range = expansion-initializer ;

Unfortunately, the last paragraph of bullet 5.3 does not cover this declaration, therefore it is unclear in which scenario the constexpr keyword should be present.

Suggested resolution:
In the last paragraph of bullet 5.3, include this declaration as well:
... The keyword constexpr is present in the declaration of range and the structured-binding-declaration of u₀, u₁, . . . , u_N−1 if and only if ...