Re-revert r86040, which was un-reverted in r186199.

This breaks the build of basic patterns with repeated friend
declarations. See the added test case in SemaCXX/friend.cpp or the test
case reported to the original commit log.

Original commit log:
  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: http://llvm.org/svn/llvm-project/cfe/trunk@186331 91177308-0d34-0410-b5e6-96231b3b80d8