Address http://bugs.llvm.org/pr30994 so that a non-friend can properly replace a...

Address http://bugs.llvm.org/pr30994 so that a non-friend can properly replace a friend, and a visible friend can properly replace an invisible friend but not vice verse, and definitions are not replaced. This fixes the two FIXME in SemaTemplate/friend-template.cpp.

The code implements Richard Smith suggestion in comment 3 of the PR.

reviewer: Vassil Vassilev

Differential Revision: https://reviews.llvm.org/D31540

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300443 91177308-0d34-0410-b5e6-96231b3b80d8