gccrs: Ensure uniqueness on Path probe's

gccrs: Ensure uniqueness on Path probe's

When we lookup names in paths such as Foo::bar, foo is a type we resolve
and then we lookup 'bar' based on what type Foo is which includes probing
relevant bounds of this type. We currently return a vector of possible
candidates and this patch changes it so that we return a set of unique
items based on DefId.

Addresses #1555

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc
	(CompileExpr::resolve_method_address): Use auto and minor change
	in candidate init.
	* typecheck/rust-hir-type-check-path.cc
	(TypeCheckExpr::resolve_segments): Likewise.
	* typecheck/rust-hir-type-check-type.cc: Likewise.
	* backend/rust-compile-resolve-path.cc
	(HIRCompileBase::query_compile): Likewise. Removecall to
	set_ty_ref.
	* typecheck/rust-hir-path-probe.h (struct PathProbeCandidate): Add
	locus initializer in ctor, implement get_defid.
	(class PathProbeType::Probe): return a set instead of vector.
	Adjust class impl.
	(class ReportMultipleCandidateError): Do not inherit from
	HIRImplVisitor anymore and remove corresponding impl. Adjust for
	change in Probe. Simplify Report handling.
	(class PathProbeImplTrait::Probe): Adjust return type.