Auto merge of #52332 - zackmdavis:dead_code_lint_should_say_2_electric_boogaloo,...

dead-code lint: say "constructed" for structs

Respectively.

This is a sequel to November 2017's #46103 / 1a9dc2e9. It had been
reported (more than once—at least #19140, #44083, and #44565) that the
"never used" language was confusing for enum variants that were "used"
as match patterns, so the wording was changed to say never "constructed"
specifically for enum variants. More recently, the same issue was raised
for structs (#52325). It seems consistent to say "constructed" here,
too, for the same reasons.

~~While we're here, we can also use more specific word "called" for unused
functions and methods. (We declined to do this in #46103, but the
rationale given in the commit message doesn't actually make sense.)~~

This resolves #52325.