[OClToSPIRV] Do not identify user defined functions as OpenCL builtins

[asudarsa]

There are scenarios where users define functions with names as 'atomic_fetch_and_sub_uint32'. Such functions are being wrongly identified as OpenCL builtins.
This change adds the following to OCLIsBuiltin(...) function
if (!F->isDeclaration()) return false;

This resolves several existing test failures.

Thanks

[asudarsa]

A better solution has been submitted here: #2525
Once approved, I will close this one.

Thanks

[CLAassistant]

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