[tool] Add note about clang version used in CI to tool readme (#7273)

This repo uses clang 15 to format files. Newer versions of clang format code differently. This PR adds a note to the tool README about this and adds sample instructions for how to install clang 15 on macOS. We might want to add instructions for other platforms.

Fixes flutter/flutter#152651

Additional context
- #7267
- #7271

script/tool/README.md

@@ -47,6 +47,8 @@ anywhere within a package).
 dart run script/tool/bin/flutter_plugin_tools.dart format --packages package_name
 ```
 
+The `flutter/packages` repository uses clang version `15.0.0` . Newer versions of clang may format code differently.
+
 ### Run the Dart Static Analyzer
 
 ```sh