Skip virtual dispatches when collecting default impl mono items

[peterphitran]

Fixes #158411.

Problem

Under eager mono item collection (-Clink-dead-code), create_mono_items_for_default_impls
walks a trait's provided methods for each impl. When a trait is implemented for (something that
normalizes to) its own trait object type, e.g. impl Trait for dyn Trait spelled through a
lazy_type_alias or an associated type projection, resolving a provided method with the trait
object as the Self type yields an InstanceKind::Virtual rather than a concrete Item.

That virtual dispatch was pushed as a mono item and later fed to instance_mir, which bugs:

error: internal compiler error: virtual dispatches have no instance MIR
...
query stack during panic:
#0 [items_of_instance] collecting items used by `<dyn Trait as Trait>::a - virtual#3`
#1 [collect_and_partition_mono_items]
#2 [exported_generic_symbols]

These impls are accepted by coherence (only the syntactic impl Trait for dyn Trait is rejected
with E0371; the alias and projection forms are allowed, and are relied upon by the run-pass test
tests/ui/traits/object/ambiguity-vtable-segfault.rs), so the program should compile.

Fix

Skip the item when the resolved instance is a virtual dispatch. A virtual dispatch has no
standalone MIR body to codegen, so there is nothing to collect. This mirrors the normal
use-collection path visit_instance_use, which already skips Virtual/Intrinsic instances.

Tests

tests/ui/codegen/mono-item-collector-dyn-self-default-impl.rs, a build-pass test compiled with
-Clink-dead-code, covering:

• a lazy_type_alias self type (the original report),
• an associated type projection self type (this form reproduces with no feature gate),
• an impl with an overridden method alongside an inherited provided one, and
• an auto-trait object type (dyn Qux + Send).

[rustbot]

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[rustbot]

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

[rust-bors]

☔ The latest upstream changes (presumably #159407) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.