The incompetent fool who added these suggestions in 38e5a964f2 apparently
thought it was safe to assume that, because the offending function or
static was unreachable, it would therefore have not have any existing
visibility modifiers, making it safe for us to unconditionally suggest
inserting `pub`. This isn't true.
This resolves#47383.
This updates the borrowck query to return a result, and this result is then used
to incrementally check for unused mutable nodes given sets of all the used
mutable nodes.
Closes#42384
