This patch changes the grouping of AS so that:

  Foo AS WITH bar AS 1 SELECT 2

with no longer be grouped as:

  [Identifier[Foo, AS, WITH, Identifier[Bar AS 1]], SELECT, 2]

but will be grouped as:

  [Identifier[Foo], AS, WITH, Identifier[Bar AS 1], SELECT, 2]

This fixes the parsing of CREATE TABLE new_table AS WITH ... so the
rest of the tokens after AS are parsed the same as a bare WITH.

This change adopts some parts of the pull request #509 by
john-bodley. Thanks!