Apply sysroot to system c++ headers too. Fixes PR9372.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126884 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/InitHeaderSearch.cpp b/lib/Frontend/InitHeaderSearch.cpp
index 2a27fb1..dc40784 100644
--- a/lib/Frontend/InitHeaderSearch.cpp
+++ b/lib/Frontend/InitHeaderSearch.cpp
@@ -113,7 +113,7 @@
   llvm::StringRef MappedPathStr = Path.toStringRef(MappedPathStorage);
 
   // Handle isysroot.
-  if (Group == System && !IgnoreSysRoot &&
+  if ((Group == System || Group == CXXSystem) && !IgnoreSysRoot &&
       llvm::sys::path::is_absolute(MappedPathStr) &&
       IsNotEmptyOrRoot) {
     MappedPathStorage.clear();