[Modules] Enable local submodule visibility for ObjC/C

Remove the restriction where this is only valid with C++

rdar://problem/29055656

Differential Revision: https://reviews.llvm.org/D31781

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300108 91177308-0d34-0410-b5e6-96231b3b80d8