[Modules] Improve diagnostics for incomplete umbrella

One of the -Wincomplete-umbrella warnings diagnoses when a header is present in
the directory but it's not present in the umbrella header. Currently, this
warning only happens on top level modules; any submodule using an umbrella
header does not get this warning. Fix that by also considering the submodules.

Differential Revision: https://reviews.llvm.org/D32576

rdar://problem/22623686

llvm-svn: 301597