Allow submodule inferrences with a missing umbrella when the module is unavailable

If the module is unavailable because of a missing header, don't diagnose
a "module * {}" declaration for having a missing umbrella.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@206776 91177308-0d34-0410-b5e6-96231b3b80d8