Make break and continue hygienic

Makes labelled loops hygiene by performing renaming of the labels
defined in e.g. `'x: loop { ... }` and then used in break and continue
statements within loop body so that they act hygienically when used with
macros.

Closes #12262.

src/librustc/middle/cfg/construct.rs

@@ -490,7 +490,7 @@ impl CFGBuilder {
 
     fn find_scope(&self,
                   expr: @ast::Expr,
-                  label: Option<ast::Name>) -> LoopScope {
+                  label: Option<ast::Ident>) -> LoopScope {
         match label {
             None => {
                 return *self.loop_scopes.last().unwrap();