Simplify parsing struct field declarators This removes the dummy type that was attached to struct declarators while parsing. This makes TParseContext::addStructDeclaratorList in particular simpler to understand. The new TDeclarator data type is the parsed representation of the struct_declarator grammar rule. It is completely immutable. The name and location stored in TField can also be qualified as constant now. BUG=angleproject:2267 TEST=angle_unittests Change-Id: I2834f87fc0eee0bdb7673ef495a55fb463023c55 Reviewed-on: https://chromium-review.googlesource.com/797033 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>