[modules] Add support for 'textual header' directives.

This allows a module to specify that it logically contains a file, but that
said file is non-modular and intended for textual inclusion. This allows
layering checks to work properly in the presence of such files.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220448 91177308-0d34-0410-b5e6-96231b3b80d8