Let Clang provide image access qualifiers.

The previous solution already depended on this because
QualType::getAsString() printed qualifiers only if Clang supported
them.

We are now depending on the Clang patch that introduces image access
qualifier supported. Therefore this commit may need to be fixed when
validator is compiled against upstream Clang.

Signed-off-by: Rami Ylimäki <rami.ylimaki@vincit.fi>