Introduce -runtime-compatibility-version flag as a no-op.

Stage in part of swiftlang#25030 so that build systems can start using it.

Author: jckarter

include/swift/Option/Options.td

@@ -883,4 +883,8 @@ def vfsoverlay : JoinedOrSeparate<["-"], "vfsoverlay">,
 def vfsoverlay_EQ : Joined<["-"], "vfsoverlay=">,
   Alias<vfsoverlay>;
 
+// Runtime compatibility version
+def runtime_compatibility_version : Separate<["-"], "runtime-compatibility-version">,
+  Flags<[FrontendOption]>,
+  HelpText<"Link compatibility library for Swift runtime version, or 'none'">;
 include "FrontendOptions.td"