fix Issue 12223 - __traits(getMember,...) needed for aliases

This is my attempt at implementing [issue 12223](https://d.puremagic.com/issues/show_bug.cgi?id=12223), a `__traits` primitive to evaluate a symbol with a given `this`.

~~This implementation works - mostly. However, it doesn't work when chaining the `child` trait - the `DotExp` never seems to get lowered, and the backend chokes on it. See the commented out lines in the test for an example. Perhaps someone can give me a hint at what am I doing wrong?~~

Will do a doc pull when this is merged.