Don't build modules with (submodules with) missing headers

Unless they are in submodules that aren't available anyway, due to
requirements not being met.  Also, mark children as unavailable when the
parent is.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206664 91177308-0d34-0410-b5e6-96231b3b80d8