Improve the ICE message for invalid nullary intrinsic calls
In https://github.com/rust-lang/rust/issues/148104, we found the panic message here rather confusing, and (if I'm reading the tea leaves right) that's because the intended audience for either side of the phrase is very different. I think this is more clear if/when this is encountered by users.
I expect this ICE to be hit in practice by people calling the `size_of` and `align_of` intrinsics, so it's now _kind of_ helpful for those users too.
The original effort to stop backends from needing to support nullary intrinsics added a note to all these const-only intrinsics, but when https://github.com/rust-lang/rust/pull/147793 ported two more the paragraph wasn't added. I've added it.
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