gccrs: support use declaration to write the type into the correct namespace

This builds upon the previous path resolution fixes so that it returns the
resolved_node_id or UNKNOWN_NODEID on failure to resolve the use-path.
It then exports the name to the current module namespace so it can be used.

Fixes #850 #855

gcc/rust/ChangeLog:

	* ast/rust-ast.h: add const get_final_segment helper
	* hir/rust-ast-lower-enumitem.h: dont add an item mapping for enumitems
	* hir/rust-ast-lower-item.cc (ASTLoweringItem::visit): add enum to enum-items mappings
	* hir/rust-ast-lower-stmt.cc (ASTLoweringStmt::visit): likewise
	* hir/tree/rust-hir-item.h: add non const helper to get variants
	* resolve/rust-ast-resolve-item.cc (ResolveItem::visit): resolve the use declaration
	* resolve/rust-ast-resolve-path.cc (ResolvePath::resolve_path): handle self
	* resolve/rust-ast-resolve-toplevel.h: add enum item mappings to module mappings
	* typecheck/rust-hir-type-check-path.cc (TypeCheckExpr::resolve_root_path): ensure variant
	(TypeCheckExpr::resolve_segments): likewise
	* typecheck/rust-type-util.cc (query_type): lookup enum's
	* util/rust-hir-map.cc (Mappings::insert_hir_enumitem): enum item mappings
	(Mappings::lookup_hir_enumitem): likewise
	* util/rust-hir-map.h: likewise

gcc/testsuite/ChangeLog:

	* rust/compile/issue-850.rs: New test.
	* rust/compile/issue-855.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

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