This is triggered when we are compiling an implementation of a module,
it has relative includes to a VFS-mapped module with umbrella headers.
Currently we will find the real path to headers under the umbrella directory,
but the umbrella directories are using virtual path.
rdar://27951255
Thanks Ben and Richard for reviewing the patch!
Differential Revision: http://reviews.llvm.org/D23858
llvm-svn: 279838
Manman Ren