Deprecate LLVM typed pointers

[HertzDevil]

Part of #12743. This PR adds the actual deprecation notices to methods that assume the availability of typed pointers.

[straight-shoota]

I'd suggest to maybe add to the deprecation message that these deprecations apply for LLVM 15+.
Or actually, do I understand correctly, that these deprecated methods already won't work when targeting LLVM 15+? So it's not just deprecated, it's already broken for recent LLVM?

[HertzDevil]

The LLVMBuild* funs are deprecated in LLVM 15, and not exported at all in 16+. It seems LLVMContextSetOpaquePointers is exported in both 15 and 16, but that doesn't matter to us.

[straight-shoota]

Okay, then I suppose for LLVM 16 support we should add compile time errors to these methods that indicate that they're not supported with LLVM 16+.
For now (including LLVM 15) it's fine to just have deprecations.
We could consider making the deprecations actually dependent on the LLVM version as well (only mark them as deprecated when targeting LLVM 15 because they're technically fine to use with older LLVM versions), but I don't think that makes sense. They should be deprecated in general and we advise against using them in favour of the future-proof alternative.