swift234

Parse attributes as part of <type> and simplify the grammar.

This means that we accept type attributes in a much broader
range of places where we previously required a <type>.  <type>
was already a production that demanded a grammatically
unconstrained context because of all the possible continuations;
reducing the number of independent productions makes it easier
to choose one and thus not accidentally limit the range of
possible types parsed.

In particular, we want to be able to parse @unchecked T? pretty
much anywhere you can write a type.

Swift SVN r14912