Auto merge of #34198 - eddyb:you're-a-bad-transmute-and-you-should-feel-bad, r=nikoma...

Make transmuting from fn item types to pointer-sized types a hard error.

Closes #19925 by removing the future compatibility lint and the associated workarounds.
This is a `[breaking-change]` if you `transmute` from a function item without casting first.
For more information on how to fix your code, see https://github.com/rust-lang/rust/issues/19925.

diff --cc src/librustc/lint/builtin.rs
Simple merge

diff --cc src/librustc_lint/lib.rs
Simple merge