Enable macOS GitHub Action CI (swiftlang#1311)

* Enable macOS GitHub Action CI

* Add explicit macOS version list

Author: jmschonfeld

.github/workflows/pull_request.yml

@@ -14,6 +14,9 @@ jobs:
     with:
       linux_swift_versions: '["nightly-main"]'
       windows_swift_versions: '["nightly-main"]'
+      enable_macos_checks: true
+      macos_xcode_versions: '["16.3"]'
+      macos_versions: '["sequoia"]'
 
   soundness:
     name: Soundness