translator: Fix InitializeVariables with gl_ClipDistance. The extension allows for the shader to redeclare this variable with a different size. This would sometimes put the tree in an inconsistent state when used with the InitializeVariables feature. Fix this by using the user declaration of gl_ClipDistance when available. Found when working on trace validation, where using the init output variables option in the compiler would then cause tree validation to fail for one specific test. Test: ClipDistanceTest.ThreeClipDistancesRedeclared Bug: angleproject:5133 Change-Id: I42c9bfbe313826ac5d391068d9fc18eb5793375f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3110752 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>