Recognize `cl_khr_kernel_clock` builtins and translate them to
`OpReadClockKHR` instructions. The `Scope` operand is deduced from the
builtin function name.
spirv-val does not pass yet due to OpReadClockKHR only supporting the
valid scopes for Vulkan (Device and Subgroup, but not Workgroup), so
leave validation disabled with a TODO.
89c23f7683