Fix missing -Xfrontend in test

The emitVariantModule tests had a missing `-Xfrontend` on the module doc
flag resulting in a warning from the driver about an unused input. That
is intended to be a value for the variant module doc flag, so fixing
that while we're in here.

Author: etcwilde

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -4182,7 +4182,7 @@ final class SwiftDriverTests: XCTestCase {
         "-emit-module-path", "target.swiftmodule",
         "-emit-variant-module-path", "variant.swiftmodule",
         "-Xfrontend", "-emit-module-doc-path", "-Xfrontend", "target.swiftdoc",
-        "-Xfrontend", "-emit-variant-module-doc-path", "variant.swiftdoc",
+        "-Xfrontend", "-emit-variant-module-doc-path", "-Xfrontend", "variant.swiftdoc",
         "-emit-module-source-info-path", "target.sourceinfo",
         "-emit-variant-module-source-info-path", "variant.sourceinfo",
         "-emit-package-module-interface-path", "target.package.swiftinterface",