Location and struct name matching for fields was missing as they only
apply to I/O blocks and not varyings of struct type.

Bug: angleproject:3580
Change-Id: I69083f39088458da72828b418be3068187a30fcc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2587456Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>