Always read bitcode according to the -opaque-pointers mode. Do not perform auto-detection to implicitly switch to typed pointers.
This is a step towards removing typed pointer support, and also eliminates the class of problems where linking may fail if a typed pointer module is loaded before an opaque pointer module. (The latest place where this was encountered is https://reviews.llvm.org/D139924, but this has previously been fixed in other places doing bitcode linking as well.)
PS: This currently still fails a small handful of tests, which I'll update before landing this.