Rollup merge of #88706 - ThePuzzlemaker:issue-88609, r=jackh726

Normalize associated type projections when checking return type of main

This fixes #88609.

Previously, the return type of `fn main()` would not have any associated type projections within normalized before checking if it implements the standard library trait `std::process::Termination`. This commit appears to fix it.

This feels vaguely symptomatic of a problem in the underlying trait solving engine, but I am not sure how I would solve that. I am unsure why the example in #88609 with `assert_impl_termination` and `fn foo()` work, but simply `fn main()` doesn't. The way that I solved this is also probably not the best way to do this, so please let me know if there is a better way to do this.

I have added a build-pass regression test for this issue.

src/test/ui/typeck/issue-88609.rs

@@ -0,0 +1,19 @@
+// Regression test for #88609:
+// The return type for `main` is not normalized while checking if it implements
+// the trait `std::process::Termination`.
+
+// build-pass
+
+trait Same {
+    type Output;
+}
+
+impl<T> Same for T {
+    type Output = T;
+}
+
+type Unit = <() as Same>::Output;
+
+fn main() -> Result<Unit, std::io::Error> {
+    unimplemented!()
+}