Fix Issue 13738 - RTInfo generation on deprecated struct causes deprecation message

assume aggregate to be non-deprecated during RTInfo generation

https://issues.dlang.org/show_bug.cgi?id=13738
I know doing things like this works, but in my experience tweaking global settings like this usually turn out badly, causing unexpected bugs popping up in other places. Please, there's got to be a better way!

(For a primo example, the gagging system in dmd. It's awful, and has been the cause of endless problems and bugs.)