[libc++] Work around dynamic linking of stringbuf::str() on Windows

https://github.com/llvm/llvm-project/issues/40363 caused the C++20
`str() const &` and `str() &&` to be dllimport'ed despite _LIBCPP_HIDE_FROM_ABI.
This is a temporary solution until #40363 is fixed.

Reviewed By: #libc, hans, ldionne, Mordante

Differential Revision: https://reviews.llvm.org/D155185