Placeholder object for VkPipelineCache

Placeholder object for VkPipelineCache

While implementing Pipeline Cache is optional, we still need a
placeholder object in order to return have a valid handle for
this object.

b/118386749

Change-Id: I18abb4196fbc99d3f639c1ba14ceb570ac11b365
Reviewed-on: https://swiftshader-review.googlesource.com/c/22729
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>