[mlir] Fix FunctionOpInterface extraSharedClassDeclaration to be fully namespace qualified (#82682)
`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";
}