cg_llvm: More preparation for moving FFI bindings to rustc_llvm

[Zalathar]

• Sequel to cg_llvm: Remove inherent methods from several LLVM FFI types #147327
• Motivated by Move all LLVM externs into the rustc_llvm crate #142897

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This is another batch of adjustments to code that would otherwise interfere with moving FFI bindings out of rustc_codegen_llvm and into rustc_llvm.

[rustbot]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.