fix: gallivm: limit usage of LLVMContextSetOpaquePointers() to LLVM 15

LLVMContextSetOpaquePointers() was a temporary workaround offered to
allow transitioning to opaque pointers. Mesa is ready for this now with
recent versions of LLVM (16+). Therefore we limit the workaround of
using LLVMContextSetOpaquePointers() to LLVM 15 now, that this C
interface was removed in LLVM 17.

Reference: https://github.com/llvm/llvm-project/commit/1270879376025457e266572c7cccc51eb5de9276
Suggested-by: Dave Airlie <airlied@redhat.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8183
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21124>
Change-Id: I544e7e09f234405adf53bcd4d33dcf67db68e601
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/915134
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: John Bauman <jbauman@google.com>
Fuchsia-Auto-Submit: Josh Gargus <jjosh@google.com>
7 files changed