Rollup merge of #74623 - lcnr:polymorphize-functions, r=eddyb

polymorphize GlobalAlloc::Function

this sadly does not change #74614

r? @eddyb

src/test/ui/polymorphization/promoted-function.rs

@@ -0,0 +1,13 @@
+// run-pass
+fn fop<T>() {}
+
+fn bar<T>() -> &'static fn() {
+    &(fop::<T> as fn())
+}
+pub const FN: &'static fn() = &(fop::<i32> as fn());
+
+fn main() {
+    bar::<u32>();
+    bar::<i32>();
+    (FN)();
+}