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/212937Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>