[ADT] Add resize_for_overwrite method to SmallVector.

Analagous to the std::make_(unqiue|shared)_for_overwrite added in c++20.
If T is POD, and the container gets larger, any new values added wont be initialized.
This is useful when using SmallVector as a buffer where its planned to overwrite any potential new values added.
If T is not POD, `new (Storage) T` functions identically to `new (Storage) T()` so this will function identically to `resize(size_type)`.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D93532