In preparation for support for mid-block location qualifiers.

Bug: angleproject:3580
Change-Id: Ia63f20c8dc26e8f0e90371c4f00913585a22267a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2577983Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>