gccrs: ast: Refactor how lang item paths are handled.

gccrs: ast: Refactor how lang item paths are handled.

Lang item typepaths were not handled properly, and required a complete overhaul.
All old classes that concerned lang item paths are now modified to use a simpler
version of `AST::LangItemPath`, which has been removed. TypePath segments can now
be lang items, as this is requied for having generic lang item paths such as
PhantomData<T>.

gcc/rust/ChangeLog:

	* ast/rust-path.h: Rework how lang item paths are represented.
	* ast/rust-path.cc: Likewise.
	* ast/rust-item.h: Likewise.
	* ast/rust-ast.cc: Likewise.
	* ast/rust-ast-collector.cc: Adapt to new lang item path system.
	* ast/rust-ast-collector.h: Likewise.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Likewise.
	* ast/rust-ast-visitor.h: Likewise.
	* expand/rust-derive-copy.cc: Likewise.
	* expand/rust-derive.h: Likewise.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::visit): Likewise.
	* hir/rust-ast-lower-base.h: Likewise.
	* hir/rust-ast-lower-type.cc (ASTLowerTypePath::translate): Likewise.
	(ASTLowerTypePath::visit): Likewise.
	* hir/rust-ast-lower-type.h: Likewise.
	* resolve/rust-ast-resolve-base.cc (ResolverBase::visit): Likewise.
	* resolve/rust-ast-resolve-base.h: Likewise.
	* resolve/rust-ast-resolve-item.cc (ResolveItem::visit): Likewise.
	* resolve/rust-ast-resolve-type.h: Likewise.
	* resolve/rust-ast-resolve-type.cc (ResolveRelativeTypePath::go): Likewise.
	* resolve/rust-late-name-resolver-2.0.cc (Late::visit): Likewise.
	* resolve/rust-late-name-resolver-2.0.h: Likewise.
	* hir/tree/rust-hir-path.cc (TypePathSegment::TypePathSegment): Likewise.
	(TypePathSegmentGeneric::TypePathSegmentGeneric): Likewise.
	* hir/tree/rust-hir-path.h: Likewise.
	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::resolve_root_path): Likewise.
	* ast/rust-ast-builder.cc: Likewise.
	* ast/rust-ast-builder.h: Likewise.