gccrs: Rename label getter to unchecked

gccrs: Rename label getter to unchecked

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Update label
	getter call.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Likewise.
	* ast/rust-ast.cc (BreakExpr::as_string): Likewise.
	* hir/rust-ast-lower-expr.cc (ASTLoweringExpr::visit): Likewise.
	* resolve/rust-ast-resolve-expr.cc (ResolveExpr::visit): Likewise.
	* resolve/rust-late-name-resolver-2.0.cc (Late::visit): Likewise.
	* ast/rust-expr.h: Add optional getter and rename label getter to
	get_label_unchecked.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>