coverage: Avoid a query stability hazard in function_coverage_map

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.

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -58,11 +58,6 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
         return;
     }
 
-    // The entries of the map are only used to get a list of all files with
-    // coverage info. In the end the list of files is passed into
-    // `GlobalFileTable::new()` which internally do `.sort_unstable_by()`, so
-    // the iteration order here does not matter.
-    #[allow(rustc::potential_query_instability)]
     let function_coverage_entries = function_coverage_map
         .into_iter()
         .map(|(instance, function_coverage)| (instance, function_coverage.into_finished()))