Backport cxx14 default dialect flag from clang 6.0.0 trunk to 5.0.1

Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- a/clang/lib/Frontend/CompilerInvocation.cpp	(revision 320871)
+++ b/clang/lib/Frontend/CompilerInvocation.cpp	(working copy)
@@ -1690,11 +1690,11 @@
       break;
     case InputKind::CXX:
     case InputKind::ObjCXX:
-      // The PS4 uses C++11 as the default C++ standard.
-      if (T.isPS4())
-        LangStd = LangStandard::lang_gnucxx11;
-      else
-        LangStd = LangStandard::lang_gnucxx98;
+#if defined(CLANG_DEFAULT_STD_CXX)
+      LangStd = CLANG_DEFAULT_STD_CXX;
+#else
+      LangStd = LangStandard::lang_gnucxx14;
+#endif
       break;
     case InputKind::RenderScript:
       LangStd = LangStandard::lang_c99;