Don't diagnose non-modular includes when we are not compiling a module.

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