Validate variable types for UniformConstant storage in Vulkan (#2008) (#2044)

Validate variable types for UniformConstant storage in Vulkan (#2008)

From the Vulkan 1.1 spec 14.5.2:
  Variables identified with the UniformConstant storage class are used
  only as handles to refer to opaque resources. Such variables must be
  typed as OpTypeImage, OpTypeSampler, OpTypeSampledImage, or an array
  of one of these types.

Fixes #2008