Use Memset<T> as the first base class of cache key types to ensure they
get initialized before any other base classes or members get
constructed.

Bug: b/134932616
Change-Id: I8f28252696d6e017db11da068180d2425cdf1d57
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33249Tested-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>