Be sure to clear all program state at unlink
Be sure to clear all program state at unlink

At Program::unlink we clear out the program's state but forgot
mBufferVariables. That was causing us to hit an assert when trying to
load a shader from cache.
Only see the issue on Android as that's the only platform where shader
caching is supported.

Test: KHR-GLES31.core.explicit_uniform_location.uniform-loc
Bug: angleproject:4132
Change-Id: Iae9c26a3e3b2d11983cc7d90bcd54a10c0fa9934
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2107235
Reviewed-by: Cody Northrop <94770d7fe1cce6ea0791fa9927c1107de4d2e37b@google.com>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <aa371ec5c940a0ded89f7ed95bce84faebcf51df@google.com>

ppy/angle,ppy/angle,ppy/angle,ppy/angle