Audit src/rustllvm/RustWrapper.cpp for C APIs that are now in upstream LLVM.

[eddyb]

Specifically, functions which are implemented in the oldest supported LLVM version.

• LLVMRustSetComdat => Comdat.h compiler: Use LLVM's Comdat support #131876
• LLVMRustModuleBuffer => Core.h::LLVMMemoryBufferRef
• LLVMRustDI* => Debuginfo.h::LLVMDI*

We can also consider to polyfill some functions that are available in our fork, but not in some older supported version, using the same name and signature, instead of our own.

cc @alexcrichton @nagisa @nikic @hanna-kruppe