[OpaquePtr] Remove existing support for forward compatibility

It assumes that PointerType will keep having an optional pointee type,
but we'd like to remove the pointee type in PointerType at some point.

I feel like the current implementation could be simplified anyway,
although perhaps I'm underestimating the amount of work needed
throughout BitcodeReader.

We will still need a side table to keep track of pointee types. This
will be reimplemented at some point.

This is essentially a revert of a4771e9d (which doesn't look like it was
reviewed anyway).

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D103135