Add new flag to enable experimental emit-module for the target variant from a single driver invocation

This flag will be used to control opting into behavior introduced in swiftlang/swift-driver#1856

Author: artemcm

include/swift/Option/Options.td

@@ -1571,6 +1571,10 @@ def explain_module_dependency_detailed : Separate<["-"], "explain-module-depende
 def explicit_auto_linking : Flag<["-"], "explicit-auto-linking">,
   Flags<[NewDriverOnlyOption]>,
   HelpText<"Instead of linker-load directives, have the driver specify all link dependencies on the linker invocation. Requires '-explicit-module-build'.">;
+  
+def experimental_emit_variant_module : Flag<["-"], "experimental-emit-variant-module">,
+  Flags<[NewDriverOnlyOption]>,
+  HelpText<"When a target variant triple is specified, the same driver invocation will emit two Swift modules, one for the primary target and one for the variant.">;
 
 def min_inlining_target_version : Separate<["-"], "target-min-inlining-version">,
   Flags<[FrontendOption, ModuleInterfaceOption]>,