[Clang] Add helper method to determine if a nonvirtual base has an entry in the LLVM struct

This patch adds a helper method to determine if a nonvirtual base has an entry in the LLVM struct. Such a base may not have an entry
if the base does not have any fields/bases itself that would change the size of the struct. This utility method is useful for other frontends (Polygeist) that use Clang as an API to generate code.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D122502