Unrevert r186040, reverted in r186185, with fix for PR16597.

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.

llvm-svn: 186199