[libc++] Consider adding an `allocate_at_least`-like function to the new-family of functions

Currently `std::allocator`'s `allocate_at_least` simply allocates an exact amount of memory. Instead, we could introduce a function like `allocation_result __new_at_least(size_t count)` which uses platform-specific functions to do what `allocate_at_least` is intended for: returning the actual amount of memory allocated.

This would improve performance of e.g. `vector` and `string`, since they use `allocate_at_least` internally.