Workaround OpaquePointers for LLVM 15

[toppk]

This workaround allows bpftrace to be compiled against LLVM-15. This will have to be address properly before LLVM-16
More details from LLVM here: https://llvm.org/docs/OpaquePointers.html

Open issues here:
#2366
#2365

Checklist

• Language changes are updated in man/adoc/bpftrace.adoc and if needed in docs/reference_guide.md
• User-visible and non-trivial changes updated in CHANGELOG.md
• The new behaviour is covered by tests

[hhoffstaette]

Tested (wrt. #2366) and seems to work. Thanks!

[viktormalik]

Thanks! I created the issue #2368 to track the proper fix.

[miwojcik]

Hi @viktormalik! Is this workaround going to be merged or are you working on a proper solution? I hit exactly the same issue (#2365 but on yocto/poky) and this workaround is working, but I wonder if I need to temporary backport it.

[viktormalik]

Since LLVM 15 is already out, I'd suggest to merge this workaround to have a fix until I finish the proper solution.

@danobi, @fbs, @ajor any objections?

[fbs]

no, thanks!