Don't print the same impl twice on ext. overlap

[jonas-schievink]

This makes the error message in #28981 a bit shorter (152 to 115 lines).

Previous output (the local impl was always printed twice when it conflicted with an external impl):

test.rs:3:1: 3:23 error: conflicting implementations for trait `core::ops::Deref` [E0119]
test.rs:3 impl<T> Deref for T {}
          ^~~~~~~~~~~~~~~~~~~~~~
test.rs:3:1: 3:23 help: run `rustc --explain E0119` to see a detailed explanation
test.rs:3:1: 3:23 note: conflicting implementation in crate `std`
test.rs:3 impl<T> Deref for T {}
          ^~~~~~~~~~~~~~~~~~~~~~

Output after this patch:

test.rs:3:1: 3:23 error: conflicting implementations for trait `core::ops::Deref` [E0119]
test.rs:3 impl<T> Deref for T {}
          ^~~~~~~~~~~~~~~~~~~~~~
test.rs:3:1: 3:23 help: run `rustc --explain E0119` to see a detailed explanation
note: conflicting implementation in crate `std`

[rust-highfive]

r? @Aatch

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[arielb1]

This also disables the note on same-crate, right?

[jonas-schievink]

impl2.is_local() should be true then, right? I didn't change that code path.

[Aatch]

Looks fine to me, since this is just removing the span for the note.

[Aatch]

@bors r+ 6c094b9

[bors]

⌛ Testing commit 6c094b9 with merge 7e3c8cf...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt