The 'no_undeclared_includes' attribute should be used in a module to
tell that only non-modular headers and headers from used modules are
accepted.
The main motivation behind this is to prevent dep cycles between system
libraries (such as darwin) and libc++.
Patch by Richard Smith!
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@284797 91177308-0d34-0410-b5e6-96231b3b80d8
Bruno Cardoso Lopes