Add parser support for generalized where clauses

Implement support in the parser for generalized where clauses,
as well as the conversion of ast::WherePredicates to
ty::Predicate in `collect.rs`.
Add support for multiple region bounds in where clauses
patch up pretty-printing test
auto merge of #20073 : nikomatsakis/rust/generalized-where-clause-parser, r=nikomatsakis

This is the same branch as #20002 but with the pretty-printing test fixed.