Auto merge of #74733 - richkadel:llvm-coverage-map-gen-5, r=tmandry

Fixed coverage map issues; better aligned with LLVM APIs

Found some problems with the coverage map encoding when testing with more than one counter per function.

While debugging, I realized some better ways to structure the Rust implementation of the coverage mapping generator. I refactored somewhat, resulting in less code overall, expanded coverage of LLVM Coverage Map capabilities, and much closer alignment with LLVM data structures, APIs, and naming.

This should be easier to follow and easier to maintain.

r? @tmandry

Rust compiler MCP rust-lang/compiler-team#278
Relevant issue: #34701 - Implement support for LLVMs code coverage instrumentation