WebCLAnalyser::handleVarDecl: constants now require initializer and globals must be constants

Added also simple test case variable-global-scope.el.
sampler_t: added tests for global sampler_t variables (must be constant)
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.
Merge pull request #62 from eras/global_scope_private

WebCLAnalyser:handleVarDecl: global scope private variables are now collected