If we friend a declaration twice, that should not make it visible to name

If we friend a declaration twice, that should not make it visible to name
lookup in the surrounding context. Slightly rework how we handle friend
declarations to inherit the visibility of the prior declaration, rather
than setting a friend declaration to be visible whenever there was a prior
declaration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186040 91177308-0d34-0410-b5e6-96231b3b80d8