build: add -Xfrontend to -strict-implicit-module-context

When building with the early swift driver we need to add the explicit
`-Xfrontend` to make the option take effect.

Author: compnerd

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -984,7 +984,7 @@ function(add_swift_target_library_single target name)
         -Xcc;-Xclang;-Xcc;-ivfsoverlay;-Xcc;-Xclang;-Xcc;${SWIFTLIB_SINGLE_VFS_OVERLAY})
     endif()
     list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS
-      -vfsoverlay;"${SWIFT_WINDOWS_VFS_OVERLAY}";-strict-implicit-module-context;-Xcc;-Xclang;-Xcc;-fbuiltin-headers-in-system-modules)
+      -vfsoverlay;"${SWIFT_WINDOWS_VFS_OVERLAY}";-Xfrontend;-strict-implicit-module-context;-Xcc;-Xclang;-Xcc;-fbuiltin-headers-in-system-modules)
     if(NOT CMAKE_HOST_SYSTEM MATCHES Windows)
       swift_windows_include_for_arch(${SWIFTLIB_SINGLE_ARCHITECTURE} SWIFTLIB_INCLUDE)
       foreach(directory ${SWIFTLIB_INCLUDE})
@@ -2905,7 +2905,7 @@ function(_add_swift_target_executable_single name)
 
   if(SWIFTEXE_SINGLE_SDK STREQUAL "WINDOWS")
     list(APPEND SWIFTEXE_SINGLE_COMPILE_FLAGS
-      -vfsoverlay;"${SWIFT_WINDOWS_VFS_OVERLAY}";-strict-implicit-module-context;-Xcc;-Xclang;-Xcc;-fbuiltin-headers-in-system-modules)
+      -vfsoverlay;"${SWIFT_WINDOWS_VFS_OVERLAY}";-Xfrontend;-strict-implicit-module-context;-Xcc;-Xclang;-Xcc;-fbuiltin-headers-in-system-modules)
   endif()
 
   if ("${SWIFTEXE_SINGLE_SDK}" STREQUAL "LINUX")