Tracking issue: --persist-doctests flag for rustdoc

[QuietMisdreavus]

First implemented in #56189, this flag allows users to save the compiled binaries created when rustdoc runs documentation tests.

Pending work:

• Land rustdoc: Add option to persist doc test executables #56189
• Change the folder naming scheme? i.e. rustdoc: Add option to persist doc test executables #56189 (comment)
• Related option to also save the processed code, for debugging purposes?
• Ability to cross-compile doctests and either save the binaries or run them through some test runner
• Combining the binaries into a singular test runner, like how libtest saves a single binary that runs unit tests

[jonhoo]

This would be really handy to have for getting code coverage for doctests. @QuietMisdreavus do you know what is left of the process to land this? Even in its current form it is very useful.