[Bitcode] Improve support for opaque-pointer bitcode upgrade

This is step two of supporting autoupgrade of old bitcode to opaque
pointers. Rather than tracking the element type ID of pointers in
particular, track all type IDs that a type contains. This allows us
to recover the element type in more complex situations, e.g. when
we need to determine the pointer element type of a vector element
or function type parameter.

Differential Revision: https://reviews.llvm.org/D119339