Support langkind_cxx_pch when determining the language dialect.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67315 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Driver/clang.cpp b/Driver/clang.cpp
index 48ae221..a2ec08a 100644
--- a/Driver/clang.cpp
+++ b/Driver/clang.cpp
@@ -583,6 +583,7 @@
       break;
     case langkind_cxx:
     case langkind_cxx_cpp:
+    case langkind_cxx_pch:
     case langkind_objcxx:
     case langkind_objcxx_cpp:
     case langkind_objcxx_pch: