Consider `tests/ui/const-generics/generic_const_exprs/issue-102768.stderr`,
the error message where it gives additional notes about where the associated
type is defined, and how the dead code lint doesn't have an article,
like in `tests/ui/lint/dead-code/issue-85255.stderr`. They don't have
articles, so it seems unnecessary to have one here.
Migrate mir_build's borrow conflicts
This also changes the error message slightly, for two reasons:
- I'm not a fan of saying "value borrowed, by `x`, here"
- it simplifies the error implementation significantly.
