gccrs: Use a loop label in break expression

gccrs: Use a loop label in break expression

Break expression were using a raw lifetime value instead of a loop label
this behavior would have lead to some errors in ast validation.

gcc/rust/ChangeLog:

	* ast/rust-expr.h (class BreakExpr): Change Lifetime to LoopLabel.
	* hir/rust-ast-lower-expr.cc (ASTLoweringExpr::visit): Lower lifetime
	inside the label instead.
	* resolve/rust-ast-resolve-expr.cc (ResolveExpr::visit): Resolve the
	inner lifetime.

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