Should we move the resowner field from JitContext to LLVMJitContext?

Hi,

I am implementing my own JIT plugin (based on Cranelift) for PostgreSQL 
to learn more about the JIT and noticed an API change in PostgreSQL 17.

When Heikki made the resource owners extensible in commit 
b8bff07daa85c837a2747b4d35cd5a27e73fb7b2 the API for JIT plugins changed 
when ResourceOwnerForgetJIT() was moved from the generic JIT code to the 
LLVM specific JIT code so now the resowner field of the context is only 
used by the code of the LLVM plugin.

Maybe a bit late in the release cycle but should we make the resowner 
field specific to the LLVM code too now that we already are breaking the 
API? I personally do not like having a LLVM JIT specific field in the 
common struct. Code is easier to understand if things are local. Granted 
most JIT engines will likely need similar infrastructure but just 
providing the struct field and nothing else does not seem very helpful.

See the attached patch.

Andreas