Pass -warn-on-potentially-unavailable-enum-case frontend flag when building a module

Fixes rdar://problem/78306593.

Author: slavapestov

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -85,6 +85,13 @@ extension Driver {
       commandLine.appendFlag("-aarch64-use-tbi")
     }
 
+    // Potentially unavailable enum cases are downgraded to a warning when building a
+    // swiftmodule, to allow building a module (or module interface) for an older
+    // deployment target than the framework itself.
+    if compilerOutputType == .swiftModule {
+      commandLine.appendFlag("-warn-on-potentially-unavailable-enum-case")
+    }
+
     // Enable or disable ObjC interop appropriately for the platform
     if targetTriple.isDarwin {
       commandLine.appendFlag(.enableObjcInterop)