The ESSL 1.00 spec states "...the extension directives must occur before
any non-preprocessor tokens." This change makes the occurance of an
extension directive after non-preprocessor tokens a compilation error.

Bug: angleproject:3285
Change-Id: I9f8631366151c92fb6d8b4408596917012d9a3e9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1565411Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>