See [LWG4061](https://cplusplus.github.io/LWG/issue4061) and
[P3341R0](https://wg21.link/p3341r0). Effectively reverts commit
36ce0c3b1e581ca310ae7d0cbc6af002cc5d0251.
`libcxx/test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp`
has a `format` function that unexpectedly takes the
`basic_format_context` by value, which is made ill-formed by LWG4061.
This PR changes the function to take the context by reference.