Unify type trait parsing

Type trait parsing is all over the place at the moment with unary, binary and
n-ary C++11 type traits that were developed independently at different points
in clang's history.

There's no good reason to handle them separately -- there are three parsers,
three AST nodes and lots of duplicated handling code with slightly different
implementations and diags for each kind.

This commit unifies parsing of type traits and sets the stage for further
consolidation.

No change in behaviour other than more consistent error recovery.