swift234

Rework PatternBindingDecl to maintain a list of pattern/initexpr pairs inside of it.

Previously, a multi-pattern var/let decl like:
  var x = 4, y = 17

would produce two pattern binding decls (one for x=4 one for y=17).  This is convenient
in some ways, but is bad for source reproducibility from the ASTs (see, e.g. the improvements
in test/IDE/structure.swift and test/decl/inherit/initializer.swift).

The hardest part of this change was to get parseDeclVar to set up the AST in a way
compatible with our existing assumptions. I ended up with an approach that forms PBDs in 
more erroneous cases than before.  One downside of this is that we now produce a spurious
  "type annotation missing in pattern"
diagnostic in some cases.  I'll take care of that in a follow-on patch.





Swift SVN r26224