This patch intends to fix an error in translating built-in interface
block fields. Any field of a built-in interface block should be kept
and cannot be hashed.

This patch can fix a bug in handling the interface block gl_in when
we try to output the translated geometry shader string.

BUG=angleproject:1941
TEST=angle_unittest
Change-Id: Iebfba4b6a30c8942ed0f66131ad30d12ad96c62a
Reviewed-on: https://chromium-review.googlesource.com/719454
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>