Change:span<const T> is constructible from initializer_list<T>.
Rationale: Permit passing a braced initializer list to a function taking span.
Effect on original feature: Valid C++ 2023 code that relies on the lack of this constructor
may refuse to compile, or change behavior in this revision of C++.
[Example 1: void one(pair<int, int>); // #1void one(span<constint>); // #2void t1(){ one({1, 2}); }// ambiguous between #1 and #2; previously called #1void two(span<constint, 2>);
void t2(){ two({{1, 2}}); }// ill-formed; previously well-formedvoid*a[10];
int x = span<void*const>{a, 0}.size(); // x is 2; previously 0
any b[10];
int y = span<const any>{b, b +10}.size(); // y is 2; previously 10 — end example]