Bug 24662 – Add a way to pass mixin arguments to pragma(msg) before mixing in

There should be a way to quickly make a `mixin` expression/declaration/statement be printed when generated. Essentially, the arguments to `mixin` are passed to `pragma(msg)` before. It’s annoying to duplicate the `mixin` arguments or have them passed to `text` and stored in an `enum` just for debugging. My sense is that `mixin debug` would work as a two-keyword keyword. (Note that `debug mixin` already has a meaning for declarations and statements.)

You can inspect the mixin output with the `-mixin=mymixins.d` switch

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