Add another workaround for -include.

 - If we don't find a file looking relative to the current working
   directory, fall back to header search. This is closer to what would
   happen if the lookup was starting from right directory in the first
   place (except it will find files in the directory of the main
   source file, which I *think* should not be found).

 - PR3992.

llvm-svn: 69794