Add language-mode option with alias to swift-version

Author: dempseyatgithub

include/swift/Option/Options.td

@@ -286,6 +286,13 @@ def swift_version : Separate<["-"], "swift-version">,
   HelpText<"Interpret input according to a specific Swift language version number">,
   MetaVarName<"<vers>">;
 
+def language_mode : Separate<["-"], "language-mode">,
+  Flags<[FrontendOption, ModuleInterfaceOption, SwiftSymbolGraphExtractOption,
+         SwiftAPIDigesterOption]>,
+  HelpText<"Interpret input according to a specific Swift language mode">,
+  MetaVarName<"<mode>">,
+  Alias<swift_version>;
+
 def package_description_version: Separate<["-"], "package-description-version">,
   Flags<[FrontendOption, HelpHidden, ModuleInterfaceOption]>,
   HelpText<"The version number to be applied on the input for the PackageDescription availability kind">,