iwyu.yml: generate and use macOS mapping file

Author: firewave

.github/workflows/iwyu.yml

@@ -101,6 +101,7 @@ jobs:
 
       - name: Build macOS mappings
         run: |
+          set -x
           wget https://raw.githubusercontent.com/include-what-you-use/include-what-you-use/master/mapgen/iwyu-mapgen-apple-libc.py
           python3 iwyu-mapgen-apple-libc.py $(xcrun --show-sdk-path)/usr/include > macos.imp