Add a getNamedVariable helper method to TParseContext. This method will be useful in more than one place, since we need to get a TType from an indentifier when parsing invariant declarations. BUG=angle:711 Change-Id: I1c1befbdcc93ea626428fb4e313b8c6326c158f6 Reviewed-on: https://chromium-review.googlesource.com/212937 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nicolas Capens <capn@chromium.org>