cg_llvm: Use some LLVM-C functions that require LLVM 22

[Zalathar]

• Follow-up to Update the minimum external LLVM to 22 #163572

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These LLVM-C functions were introduced in LLVM 22, so now that that's our baseline, we can bind to the directly and remove our own C++ wrappers.

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[mejrs]

LLVMGetOrInsertFunction has been implemented in #163690

[rust-bors]

☔ The latest upstream changes (presumably #163695) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.