Auto merge of #61735 - eddyb:must-use-life, r=oli-obk

Add deny(unused_lifetimes) to all the crates that have deny(internal).

@Zoxc brought up, regarding #61722, that we don't force the removal of unused lifetimes.
Turns out that it's not that bad to enable for compiler crates (I wonder why it's not `warn` by default?).

I would've liked to enable `single_use_lifetimes` as well, but #53738 makes it unusable for now.

For the `rustfmt` commit, I used rust-lang/rustfmt#1324 (comment), and manually filtered out some noise.

r? @oli-obk cc @rust-lang/compiler

Author: bors

src/libarena/lib.rs

@@ -13,6 +13,7 @@
 
 #![deny(rust_2018_idioms)]
 #![deny(internal)]
+#![deny(unused_lifetimes)]
 
 #![feature(core_intrinsics)]
 #![feature(dropck_eyepatch)]

src/libfmt_macros/lib.rs

@@ -10,6 +10,7 @@
 
 #![deny(rust_2018_idioms)]
 #![deny(internal)]
+#![deny(unused_lifetimes)]
 
 #![feature(nll)]
 #![feature(rustc_private)]