[mlir] Fix FunctionOpInterface extraSharedClassDeclaration to be fully namespace qualified

`extraSharedClassDeclaration` of `FunctionOpInterface` can be inherited by other
`OpInterfaces` into foreign namespaces, thus types must be fully
qualified to prevent compiler errors, for example:

    def MyFunc : OpInterface<"MyFunc", [FunctionOpInterface]> {
        let cppNamespace = "::MyNamespace";
    }

Author: shkoo