illegal-image2d-access.cl: global 'const' should be 'constant'

This so that the value is placed to constant address space.
WebCLTransformer::rewriteOrForwardSamplerArgument: CLK_ADDRESS_NONE is not a valid address

Cherry-picked a change that appears to have lost in merges.

CLK_ADDRESS_NONE is not a valid address mode per GitHub issue #43.