Temporary bootstrapping hack: introduce syntax for r egion bounds like `'b:'a`,
meaning `'b outlives 'a`. Syntax currently does nothing but is needed for full
fix to #5763. To use this syntax, the issue_5763_bootstrap feature guard is
required.
auto merge of #16318 : nikomatsakis/rust/issue-5723-bootstrap-2, r=pnkfelix

Introduce syntax for lifetime bounds like `'b:'a`, meaning `'b outlives 'a`. Syntax currently does nothing but is needed for full fix to #5763. To use this syntax, the issue_5763_bootstrap feature guard is required.