Add --no-link option

Adds the --no-link option which outputs the compiled shader binaries
without linking them. This is a first step towards allowing users to
create SPIR-v binary, non-executable libraries.

When using the --no-link option, all functions are decorated with the
Export linkage attribute.

glslang/HLSL/hlslAttributes.cpp

@@ -101,6 +101,8 @@ namespace glslang {
 
             if (name == "nonwritable")    return EatNonWritable;
             if (name == "nonreadable")    return EatNonReadable;
+
+            if (name == "export")         return EatExport;
         } else if (nameSpace.size() > 0)
             return EatNone;