Normalize assoc types when checking ret ty 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.

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