string: fix constexpr compliance of basic_string default construction#1273
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Replace raw pointer write `*ptr = 0` with `std::construct_at(ptr, char_type{})`
in both `reset_imp()` and `string_allocate_init()`. In C++20 constexpr dynamic
allocation, storage returned by `operator new` / `allocate_at_least` has not had
its lifetime started; writing through the pointer without `std::construct_at`
is ill-formed in a constant expression (Clang rejects it outright).
Add constexpr_construct.cc test verifying `u8string{}` compiles and evaluates
as a constant expression.
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Replace raw pointer write
*ptr = 0withstd::construct_at(ptr, char_type{})in bothreset_imp()andstring_allocate_init(). In C++20 constexpr dynamic allocation, storage returned byoperator new/allocate_at_leasthas not had its lifetime started; writing through the pointer withoutstd::construct_atis ill-formed in a constant expression (Clang rejects it outright). Add constexpr_construct.cc test verifyingu8string{}compiles and evaluates as a constant expression.