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>