Suggestion: "input-language" as language variable inside passes

[schuster]

Whenever I use with-output-language in a pass, it is almost always with the input language for that pass. That means whenever I change the order of my passes or add a new one in between some passes, I need to rename all the languages used in those with-output-language invocations, too. It would be nice to have something like an input-language variable that always refers to the input language for the current pass so that such renamings are unnecessary.