Fix the default libc++ header search path to be sysrooted.  Radar 9639692.

The -cxx-isystem path is not prefixed with the sysroot directory, so it's
not a good way for the driver to set the system default C++ search path.
Instead, add -stdlib as a cc1 option and teach the frontend how to find the
headers.  The driver can then just pass -stdlib through to "cc1".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133547 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/InitHeaderSearch.cpp b/lib/Frontend/InitHeaderSearch.cpp
index f94edc6..238ff7f 100644
--- a/lib/Frontend/InitHeaderSearch.cpp
+++ b/lib/Frontend/InitHeaderSearch.cpp
@@ -920,8 +920,12 @@
 void InitHeaderSearch::AddDefaultSystemIncludePaths(const LangOptions &Lang,
                                                     const llvm::Triple &triple,
                                             const HeaderSearchOptions &HSOpts) {
-  if (Lang.CPlusPlus && HSOpts.UseStandardCXXIncludes)
-    AddDefaultCPlusPlusIncludePaths(triple);
+  if (Lang.CPlusPlus && HSOpts.UseStandardCXXIncludes) {
+    if (HSOpts.UseLibcxx)
+      AddPath("/usr/include/c++/v1", CXXSystem, true, false, false);
+    else
+      AddDefaultCPlusPlusIncludePaths(triple);
+  }
 
   AddDefaultCIncludePaths(triple, HSOpts);