Guard emission of variant module behind a flag

Author: artemcm

Sources/SwiftDriver/Jobs/Planning.swift

@@ -184,6 +184,9 @@ extension Driver {
 
   private mutating func addVariantModuleJobs(addJobBeforeCompiles: (Job) -> Void,
                                              addJobAfterCompiles: (Job) -> Void) throws {
+    guard parsedOptions.contains(.experimentalEmitVariantModule) else {
+      return
+    }
     guard variantModuleOutputInfo != nil else {
       return
     }

Tests/SwiftDriverTests/ExplicitModuleBuildTests.swift

@@ -2796,6 +2796,7 @@ final class ExplicitModuleBuildTests: XCTestCase {
           """
         )
         var driver = try Driver(args: ["swiftc",
+                                       "-experimental-emit-variant-module",
                                        "-target", "x86_64-apple-macosx10.14",
                                        "-target-variant", "x86_64-apple-ios13.1-macabi",
                                        "-clang-target", "x86_64-apple-macosx12.14",
@@ -2900,6 +2901,7 @@ final class ExplicitModuleBuildTests: XCTestCase {
           $0.send("struct Profiler { void* ptr; };")
         }
         var driver = try Driver(args: ["swiftc",
+                                       "-experimental-emit-variant-module",
                                        "-target", "x86_64-apple-macosx10.14",
                                        "-target-variant", "x86_64-apple-ios13.1-macabi",
                                        "-clang-target", "x86_64-apple-macosx12.14",