[IR] Remove -opaque-pointers option

The test migration to opaque pointers has finished, so we can finally
drop typed pointer support from LLVM \o/

This removes the ability to disable typed pointers, as well as the
-opaque-pointers option, but otherwise doesn't yet touch any API
surface. I'll leave deprecation/removal of compatibility APIs to
future changes.

This also drops a few tests: These are either testing errors that
only occur with typed pointers, or type linking behavior that, to
the best of my knowledge, only applies to typed pointers.

Note that this will break some tests in the experimental SPIRV
backend, because the maintainers have failed to update their tests
in a reasonable time-frame, despite multiple warnings. In accordance
with our experimental target policy, this is not a blocking concern.
This issue is tracked at https://github.com/llvm/llvm-project/issues/60133.

Differential Revision: https://reviews.llvm.org/D155079