Fix a hole with nested unavailable submodules from r206664

If a module doesn't meet a requirement, neither do its submodules. If we
don't propogate that, we might think it's an error to be missing a
header in one of those submodules.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206673 91177308-0d34-0410-b5e6-96231b3b80d8

• include/clang/Basic/Module.h[diff]
• lib/Basic/Module.cpp[diff]
• test/Modules/Inputs/submodules/module.map[diff]