Add types for the low-level LLVM functions

We should add `declare function` TypeScript declarations for the low-level LLVM C API functions. That will make it easier to write lower-level code and will help avoid bugs in our own wrapper code.

The issue names no files or tests; first locate the TypeScript declarations and the low-level LLVM C API wrapper entry points. Identify the functions currently used without declarations, then add `declare function` types for the requested low-level API and verify that the wrapper code can use them without untyped calls.

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