survive -Inon-accessible-directory:

If part of the -I search path cannot be accessed (permissions),
clang will throw an error, even if the header can be found in
subsequent search path elements. This is counter productive as
the compilation will seem to have failed due to that error, even
though everyone is happy.

This is not the sys first error that needs to be suppressed, and
it might not be the last...

Author: Axel-Naumann

clang/lib/Lex/HeaderSearch.cpp

@@ -426,7 +426,9 @@ OptionalFileEntryRef HeaderSearch::getFileAndSuggestModule(
     std::error_code EC = llvm::errorToErrorCode(File.takeError());
     if (EC != llvm::errc::no_such_file_or_directory &&
         EC != llvm::errc::invalid_argument &&
-        EC != llvm::errc::is_a_directory && EC != llvm::errc::not_a_directory) {
+        EC != llvm::errc::is_a_directory &&
+        EC != llvm::errc::not_a_directory &&
+        EC != llvm::errc::permission_denied) {
       Diags.Report(IncludeLoc, diag::err_cannot_open_file)
           << FileName << EC.message();
     }