Make sure that initialized variable have correct storage class
For WebGPU and Vulkan environments, variables must have the storage
class; Output, Private, or Function, if they have an initializer.
Fixes#2071
From the Vulkan 1.1 spec 14.5.2:
Variables identified with the Uniform storage class are used to access
transparent buffer backed resources. Such variables must be typed as
OpTypeStruct, or an array of this type.
Fixes#1949
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
