[gn build] Port 52924a2

Author: llvmgnsyncbot

llvm/utils/gn/secondary/clang/lib/Driver/BUILD.gn

@@ -66,6 +66,7 @@ static_library("Driver") {
     "ToolChains/CommonArgs.cpp",
     "ToolChains/CrossWindows.cpp",
     "ToolChains/Cuda.cpp",
+    "ToolChains/Cygwin.cpp",
     "ToolChains/Darwin.cpp",
     "ToolChains/DragonFly.cpp",
     "ToolChains/Flang.cpp",