Finish support for code coverage

[TheElectronWill]

This is a follow-up of #13880

TODO:

• make it compile on latest main
• cleanup phase definition (InstrumentCoverage)
• try new, simpler strategy for handling function calls
• handle more trees
• see if something can be done about interpolated strings
• avoid global state
• test (any advice on how to replace the big "expect" file, if it's not the best solution?)

[smarter]

@TheElectronWill I upgraded your access level in the dotty repo to "Maintain", I believe this should let you push your changes directly into @ckipp01's branch for #13880 which should make cooperation easier.

[TheElectronWill]

Wow! Thank you for your trust Guillaume :)
I'll close this PR, then