Add flag _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS for basic_string ABI

Summary: This review is a mostly trivial change to use an explicit ABI flag for the unstable external template list. This follows the practice for an ABI flag per feature, and provides a spot for the rational / motivation for the flag.

Reviewers: EricWF, ldionne

Subscribers: dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75457