WIP patch for allowing COLLATE in plpgsql variable declarations

The attached code patch (seems to work, but no regression tests or doc
fixes yet) allows plpgsql variable declarations like

	declare
	  x text collate "en_US";

Such a collation spec overrides the default variable collation that is
otherwise inherited from the function's input collation.  I believe
this is essentially what we agreed to do a couple weeks ago.
Comments, objections?

			regards, tom lane