Add 'no-unused-function' to compile glslang

BUG=angleproject:2301

Change-Id: Iac741e007873fb718527514210649e6b658a63b8
Reviewed-on: https://chromium-review.googlesource.com/898394
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>