WebCLTypes: moved declRefExprViaImplicit from WebCLPass

Made also const-correct.
WebCLTransformer::wrappedDeclaration: fixed missing image2d_t qualifiers

This fixes #96.

If clang fixes the issue of QualType::getAsString() not outputting
OpenCL qualifiers, this can cause duplication of them in the output.
Merge pull request #98 from eras/erkkise/#96

Fix missing image qualifiers when generating wrapping checker code