-Znext-solver allow ExprKind::Call for not-yet defined opaques

[lcnr]

Stacked on top of #140375 and #145990. Revival of #140496.

Fixes rust-lang/trait-system-refactor-initiative#181

r? @BoxyUwU

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tidy check
Checking tidy rustdoc_json...
No error code explanation was removed!
tidy: Skipping binary file check, read-only filesystem
##[error]tidy error: /checkout/tests/ui/impl-trait/non-defining-uses/method-call-on-infer.rs:20: trailing whitespace
##[error]tidy error: /checkout/tests/ui/impl-trait/non-defining-uses/method-call-on-infer.rs:33: trailing whitespace
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (25.2)
linting python files
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added 291 packages in 37s
Running eslint on rustdoc JS files
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some tidy checks failed
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1583:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1225:29

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  local time: Fri Aug 29 09:40:50 UTC 2025
  network time: Fri, 29 Aug 2025 09:40:50 GMT
##[error]Process completed with exit code 1.

[lcnr]

The desired behavior is quite 🤷 here...

if we've got impl FnMut(u32) + FnMut(u32) we should prefer the Fn bound. We should do for call_trait { for item bound { check }}.

If we've got impl Fn(&'a str) + Fn(&'b str) or sth that should be handled fine, even for non-defining uses. Needs tests

Even if we've got impl Fn(T) as the opaque type, rn we don't use this at all, but instead lookup_method_for_operator just emits all the bounds necessary to use that method, so we then rely on incomplete item bounds jank to actually constrain the args of the call.

This is necessary to support stuff like

fn recur() -> impl Fn(fn(&str) -> usize) {
   if false {
       let x = recur();
       x(|s| s.len());
   }

   |_func| ()
}

the other issue is handling

fn recur() -> impl Sized {
   if false {
       let x = recur();
       x();
   }

   || ()
}

this should result in a "type must be known at this point error, not a "type doesn't implement fn"

I guess I am gonna block this PR on getting item bounds working as otherwise we're missing some important tests here