Auto merge of #115671 - Zalathar:mapgen, r=wesleywiser

coverage: Clean up encoding of per-function coverage mapping payloads

This PR contains several small improvements to the code in `rustc_codegen_llvm::coverageinfo::mapgen` that prepares a function's coverage mappings for FFI, and passes them over to LLVM to be encoded into a vector of bytes.

These changes are in preparation for some future changes to the coverage implementation, but they should all stand on their own as worthwhile.

There shouldn't be any changes to the resulting coverage mappings, as verified by the existing `tests/coverage-map` and `tests/run-coverage` suites.

The changes are mostly independent of each other, though they are indirectly affected by the indentation changes made when introducing `GlobalFileTable`.

Author: bors