[lldb] Forcefully complete a type when adding typedefs

This is very similar to D85968, only more elusive to since we were not
adding the typedef type to the relevant DeclContext until D86140, which
meant that the DeclContext was populated (and the relevant assertion
hit) only after importing the type into the expression ast in a
particular way.

I haven't checked whether this situation can be hit in the gmodules
case, but my money is on "yes".

Differential Revision: https://reviews.llvm.org/D86216