change type of ExprLoop and ExprBreak elts from ident->name.

Lots of downstream changes in librustc, should be infinitesimally faster.
added test case
auto merge of #9103 : jbclements/rust/let-var-hygiene, r=erickt

update AST so that ExprBreak and ExprCont expressions contain names, not idents. Fixes #9047 and makes progress on #6993. Simplifies the compiler very slightly, should make it (infinitesimally) faster.