Auto merge of rust-lang#124399 - ZhuUx:split-mcdc, r=Zalathar

Split mcdc code to a sub module of coverageinfo

A further work from rust-lang#124217 . I have made relatively large changes when working on rust-lang#124278 so that it would better split them from `coverageinfo.rs` to avoid potential troubling merge work with improved branch coverage by `@Zalathar` .

Besides `BlockMarkerGenerator` is added to avoid ownership problems (mostly needed for following change of rust-lang#124278 )

All code changes are done in [a37d737a](rust-lang@a3d737a) while the second commit just renames the file.

cc `@RenjiSann` `@Zalathar`
This will impact your current work.

Author: bors