[libc++] Eliminate extra allocations from `std::move(oss).str()` (#67294)

Add test coverage for the new behaviors, especially to verify that the
returned string uses the correct allocator.
Fixes https://github.com/llvm/llvm-project/issues/64644

Migrated from https://reviews.llvm.org/D157776 — @philnik777  @pfusik 
@ldionne @mordante 
 please take another look!