Auto merge of #123071 - rcvalle:rust-cfi-fix-method-fn-ptr-cast, r=compiler-errors

CFI: Fix methods as function pointer cast

Fix casting between methods and function pointers by assigning a secondary type id to methods with their concrete self so they can be used as function pointers.

This was split off from #116404.

cc `@compiler-errors` `@workingjubilee`

tests/ui/sanitizer/cfi-method-fn-ptr-cast.rs

@@ -0,0 +1,23 @@
+// Verifies that casting a method to a function pointer works.
+//
+// FIXME(#122848): Remove only-linux when fixed.
+//@ only-linux
+//@ needs-sanitizer-cfi
+//@ compile-flags: -Clto -Copt-level=0 -Cprefer-dynamic=off -Ctarget-feature=-crt-static -Zsanitizer=cfi
+//@ run-pass
+
+trait Trait1 {
+    fn foo(&self);
+}
+
+struct Type1;
+
+impl Trait1 for Type1 {
+    fn foo(&self) {}
+}
+
+fn main() {
+    let type1 = Type1 {};
+    let f = <Type1 as Trait1>::foo;
+    f(&type1);
+}