build: use C++14 for Windows builds

The Windows SDK headers require C++14 or newer features.  Visual Studio
is lax in enabling these features unlike clang.  Explicitly upgrade to
the higher standard for the Windows cross-compilation.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -856,6 +856,9 @@ function(_add_swift_library_single target name)
   if("${SWIFTLIB_SINGLE_SDK}" STREQUAL "WINDOWS")
     swift_windows_include_for_arch(${SWIFTLIB_SINGLE_ARCHITECTURE} SWIFTLIB_INCLUDE)
     target_include_directories("${target}" SYSTEM PRIVATE ${SWIFTLIB_INCLUDE})
+    set_target_properties(${target}
+                          PROPERTIES
+                            CXX_STANDARD 14)
   endif()
 
   if("${SWIFTLIB_SINGLE_SDK}" STREQUAL "WINDOWS" AND NOT "${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")