When #118865 started enforcing the `rustc::potential_query_instability` lint in
`rustc_codegen_llvm`, it added an exemption for this site, arguing that the
entries are only used to create a list of filenames that is later sorted.
However, the list of entries also gets traversed when creating the function
coverage records in LLVM IR, which may be sensitive to hash-based ordering.
This patch therefore changes `function_coverage_map` to use `FxIndexMap`, which
should avoid hash-based instability by iterating in insertion order.
5e7c1b93ac