Move `__quoted_output_proxy` into the one file that uses it.
A `const char*` has no associated traits class, so `std::quoted("literal")`
should be printable into any basic_ostream regardless of traits.
Use hidden-friend `operator<<` and `operator>>`, since we're permitted to.
(The exact signature is unspecified because the class itself is unspecified.)
We shouldn't support `std::quoted("literal")` in C++03 or C++11 mode.
(We do need `std::__quoted(s)` and `std::__quoted(cs)` in C++11 mode,
because they're used by `std::__fs::filesystem::path`.)
Differential Revision: https://reviews.llvm.org/D120135
Arthur O'Dwyer