ModuleInterface: move -user-module-version to a new field

Titled as "// swift-module-flags-ignorable:", this new field contains new
frontend arguments that can be safely ignored by the older version of the compiler.
For compilers that don't know the field at all, all arguments in it are ignored.

rdar://78233352

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -56,6 +56,10 @@ static void printToolVersionAndFlagsComment(raw_ostream &out,
       << ToolsVersion << "\n";
   out << "// " SWIFT_MODULE_FLAGS_KEY ": "
       << Opts.Flags << "\n";
+  if (!Opts.IgnorableFlags.empty()) {
+    out << "// " SWIFT_MODULE_FLAGS_IGNORABLE_KEY ": "
+        << Opts.IgnorableFlags << "\n";
+  }
 }
 
 std::string