[lldb] Use forward type in pointer-to-member

This change is similar in spirit to the change at:
https://reviews.llvm.org/rG34c697c85e9d0af11a72ac4df5578aac94a627b3

It fixes the problem where the layout of a type was being accessed
while its base classes were not populated yet; which caused an
incorrect layout to be produced and cached.

This fixes PR50054

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D100977