Revert "Revert "[clang][ve] expect libc++ headers installed at top-level of the Clang prefix (this happens also to be the default install target)""

This reverts commit 4c13bdd.

Author: Simon Moll

clang/lib/Driver/ToolChains/VEToolchain.cpp

@@ -117,11 +117,13 @@ void VEToolChain::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
     StringRef(cl_include_dir).split(Dirs, StringRef(EnvPathSeparatorStr));
     ArrayRef<StringRef> DirVec(Dirs);
     addSystemIncludes(DriverArgs, CC1Args, DirVec);
-  } else {
-    SmallString<128> P(getDriver().ResourceDir);
-    llvm::sys::path::append(P, "include/c++/v1");
-    addSystemInclude(DriverArgs, CC1Args, P);
+    return;
   }
+
+  // libc++ installed in CMAKE_INSTALL_PREFIX by default.
+  SmallString<128> P(getDriver().ResourceDir);
+  llvm::sys::path::append(P, "/../../../include/c++/v1");
+  addSystemInclude(DriverArgs, CC1Args, P);
 }
 
 void VEToolChain::AddCXXStdlibLibArgs(const ArgList &Args,