Partially inline basic_string copy constructor in UNSTABLE

    Summary:
    This is a recommit of https://reviews.llvm.org/D73223 where the added function accidentally ended up inside an idef block.

    This change splits the copy constructor up inlining short initialization, and explicitly outlining long initialization into __init_copy_ctor_external() which is the externally instantiated slow path.

    For unstable ABI, this has the following changes:

    remove basic_string(const basic_string&)
    remove basic_string(const basic_string&, const Allocator&)
    add __init_copy_ctor_external(const value_type*, size_type)
    Quick local benchmark for Copy:

    Master
    ```
    ---------------------------------------------------------------
    Benchmark                    Time             CPU   Iterations
    ---------------------------------------------------------------
    BM_StringCopy_Empty       3.50 ns         3.51 ns    199326720
    BM_StringCopy_Small       3.50 ns         3.51 ns    1...