coverage: Never emit improperly-ordered coverage regions

If we emit a coverage region that is improperly ordered (end < start),
`llvm-cov` will fail with `coveragemap_error::malformed`, which is inconvenient
for users and also very hard to debug.

Ideally we would fix the root causes of these situations, but they tend to
occur in very obscure edge-case scenarios (often involving nested macros), and
we don't always have a good MCVE to work from. So it makes sense to also have a
catch-all check that will prevent improperly-ordered regions from ever being
emitted.

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -286,6 +286,19 @@ fn make_code_region(
         start_line = source_map.doctest_offset_line(&file.name, start_line);
         end_line = source_map.doctest_offset_line(&file.name, end_line);
     }
+
+    // If we ever emit a region that is improperly ordered (end < start),
+    // `llvm-cov` will fail with `coveragemap_error::malformed`, which is
+    // inconvenient for users and also very hard to debug.
+    let is_ordered = (start_line, start_col) <= (end_line, end_col);
+    debug_assert!(is_ordered, "{start_line}:{start_col} <= {end_line}:{end_col}");
+    if !is_ordered {
+        debug!(
+            "Skipping improperly-ordered region: {start_line}:{start_col} to {end_line}:{end_col}"
+        );
+        return None;
+    }
+
     Some(CodeRegion {
         file_name,
         start_line: start_line as u32,