[libc++]Declaring '__asign_view__' as a non noexcept function

`__assign_view__` is declared as a noexcept function in `libcxx/include/__filesystem/path.h` however internally it calls `std::basic_string<char>::basic_string<char>(std::string_view)` which is not a noexcept function this may lead to a `std::terminate()` call when allocation of a new string fails.

Fixes : https://github.com/llvm/llvm-project/issues/64858

Reviewed By: Mordante, #libc

Differential Revision: https://reviews.llvm.org/D158826