Treat visibility on an enclosing namespace as a non-explicit source of

Treat visibility on an enclosing namespace as a non-explicit source of
visibility.  Fixes PR8713.

I've disabled a test which was testing that you can #pragma pop visibility
to get out of a namespace's visibility attribute.  We should probably just
diagnose that as an error unless it's instrumental to someone's system
headers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121459 91177308-0d34-0410-b5e6-96231b3b80d8