Vulkan: Implement the Pipeline cache. This currently keeps a cache of every PSO compiled and does not trim the cache or evict old members on memory pressure. This will be done as a follow-up. Improves the speed of the Draw Call microbenchmark 50x when using a single state change. Bug: angleproject:2163 Change-Id: I2cceb38ca57ae639f36a944f4571b627481b92da Reviewed-on: https://chromium-review.googlesource.com/876954 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>