[OpaquePointers][BitcodeReader] Enable -opaque-pointers if we see an opaque pointer type

This allows us to more easily test opaque pointers e.g. in the case of
ThinLTO where we only have to pass -opaque-pointers to the frontend.

Reviewed By: #opaque-pointers, nikic

Differential Revision: https://reviews.llvm.org/D122048