Auto merge of #98919 - 5225225:stricter-invalid-value, r=RalfJung

Strengthen invalid_value lint to forbid uninit primitives, adjust docs to say that's UB

For context: rust-lang/rust#66151 (comment)

This does not make it a FCW, but it does explicitly state in the docs that uninit integers are UB.

This also doesn't affect any runtime behavior, uninit u32's will still successfully be created through mem::uninitialized.

tests/ui/uninit.rs

@@ -1,5 +1,5 @@
 #![feature(stmt_expr_attributes)]
-#![allow(clippy::let_unit_value)]
+#![allow(clippy::let_unit_value, invalid_value)]
 
 use std::mem::{self, MaybeUninit};