[OpaquePtr][LLParser] Automatically detect opaque pointers in .ll files

This allows us to not have to specify -opaque-pointers when updating
IR tests from typed pointers to opaque pointers.

We detect opaque pointers in .ll files by looking for relevant tokens,
either "ptr" or "*".

Reviewed By: #opaque-pointers, nikic

Differential Revision: https://reviews.llvm.org/D119482