[OpenCL validation] Storage class of variables decorated with BuiltIn

New rule in the environment specification:
>An OpVariable in a SPIR-V module with the BuiltIn decoration represents a built-in variable. All
built-in variables must be in the Input storage class.

Start by locating the OpenCL validation handling for OpVariable and BuiltIn decorations in SPIR-V Tools. Check how existing validation rules and tests are organized, then enforce that built-in variables use the Input storage class and add coverage showing the rule rejects other storage classes.

Written by the indexing model from the issue text.