Replace some LLVMRust wrappers with calls to the LLVM C API

[Zalathar]

This PR removes the LLVMRust wrapper functions for getting/setting linkage and visibility, and replaces them with direct calls to the corresponding functions in LLVM's C API.

To make this convenient and sound, two pieces of supporting code have also been added:

• A simple proc-macro that derives TryFrom<u32> for fieldless enums
• A wrapper type for C enum values returned by LLVM functions, to ensure soundness if LLVM returns an enum value we don't know about

In a few places, the use of safe wrapper functions means that an unsafe block is no longer needed, so the affected code has changed its indentation level.