[SPIR-V] Disable opaque pointers in relese 15

As discussed on RFC mapping into SPIR-V requires pointer being preserved in some cases: https://discourse.llvm.org/t/rfc-better-support-for-typed-pointers-in-an-opaque-pointer-world/63339/23?u=anastasiastulova

As the work is still unfinished the best approach is to continue using pointer types.

Note that this change is only planned to be committed in release 15 branch.

This fixing PR56660.

Reviewed By: jcranmer-intel

Differential Revision: https://reviews.llvm.org/D130766