[ModuleInterface] Print the public keyword on imports in Swift 6 mode

If the access-level on imports proposal is accepted as written, all
imports printed in swiftinterfaces will be `public`. Whether or not we
require the explicit `public` keyword in Swift 6 mode, printing it will
have no downside. It also goes along with the mentality that
swiftinterfaces should be more explicit than implicit.

rdar://115455383

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -348,6 +348,10 @@ static void printImports(raw_ostream &out,
         out << "@_spi(" << spiName << ") ";
     }
 
+    if (M->getASTContext().LangOpts.isSwiftVersionAtLeast(6)) {
+      out << "public ";
+    }
+
     out << "import ";
     if (Opts.AliasModuleNames &&
         AliasModuleNamesTargets.contains(importedModule->getName().str()))