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 D139924, but this has
previously been fixed in other places doing bitcode linking as well.)

Differential Revision: https://reviews.llvm.org/D139940

Since D101045, allocas are no longer required to be part of the
default alloca address space. There may be allocas in multiple
different address spaces. However, the bitcode reader would
simply assume the default alloca address space, resulting in
either an error or incorrect IR.

Add an optional record for allocas which encodes the address
space.