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
This is not really a "global" setting, but modifies the current type. I agree that the solution is non-optimal.

I tried to figure if there is a way to detect running RTInfo analysis in checkDeprecated, but one problem is that a new scope is created when evaluating template arguments (https://github.com/D-Programming-Language/dmd/blob/master/src/template.c#L5998).

The problem mentioned in the comment is: if we mute messages during analysis of RTInfo, it might silently instantiate templates that use (other) deprecated types. If these template instances are used in other parts of the program later, they will be reused without ever producing the deprecation message. The implementation here restricts muting to the types that RTInfo is currently generated for.