Rollup merge of #130252 - compiler-errors:const-gen, r=chenyukang

Properly report error on `const gen fn`

Fixes #130232

Also removes some (what I thought were unused) functions, and fixes a bug in clippy where we considered `gen fn` to be the same as `fn` because it was only built to consider asyncness.

tests/ui/coroutine/const_gen_fn.rs

@@ -0,0 +1,12 @@
+//@ edition:2024
+//@ compile-flags: -Zunstable-options
+
+#![feature(gen_blocks)]
+
+const gen fn a() {}
+//~^ ERROR functions cannot be both `const` and `gen`
+
+const async gen fn b() {}
+//~^ ERROR functions cannot be both `const` and `async gen`
+
+fn main() {}

tests/ui/coroutine/const_gen_fn.stderr

@@ -0,0 +1,20 @@
+error: functions cannot be both `const` and `gen`
+  --> $DIR/const_gen_fn.rs:6:1
+   |
+LL | const gen fn a() {}
+   | ^^^^^-^^^----------
+   | |     |
+   | |     `gen` because of this
+   | `const` because of this
+
+error: functions cannot be both `const` and `async gen`
+  --> $DIR/const_gen_fn.rs:9:1
+   |
+LL | const async gen fn b() {}
+   | ^^^^^-^^^^^^^^^----------
+   | |     |
+   | |     `async gen` because of this
+   | `const` because of this
+
+error: aborting due to 2 previous errors
+