use --std=gnu++14 when building cxx file in interop tests

Author: WANGJIEKE

test/Interop/lit.local.cfg

@@ -25,7 +25,7 @@ config.substitutions.insert(0, ('%target-interop-build-swift',
 config.substitutions.insert(0, ('%target-interop-build-clang',  '%target-clang -x c -Werror=implicit-function-declaration ' + clang_opt))
 
 # Build C++ files with matching link settings, if required by the target.
-config.substitutions.insert(0, ('%target-interop-build-clangxx', '%target-clangxx ' + clang_opt))
+config.substitutions.insert(0, ('%target-interop-build-clangxx', '%target-clangxx --std=gnu++14 ' + clang_opt))
 
 # Test parsing of the generated C++ header in different C++ language modes.
 config.substitutions.insert(0, ('%check-interop-cxx-header-in-clang\(([^)]+)\)',