Auto merge of #34923 - eddyb:deny-fn-item-transmute, r=nikomatsakis

Deny (by default) transmuting from fn item types to pointer-sized types.

This sets the #19925 lint (transmute from zero-sized fn item type) to `deny` by default.
Technically a `[breaking-change]`, but will not affect dependent crates because of `--cap-lints`.

src/librustc/lint/builtin.rs

@@ -157,7 +157,7 @@ declare_lint! {
 
 declare_lint! {
     pub TRANSMUTE_FROM_FN_ITEM_TYPES,
-    Warn,
+    Deny,
     "transmute from function item type to pointer-sized type erroneously allowed"
 }