Put friend decls in the correct context.

When we find a friend declaration we have to skip transparent contexts for doing
lookups, but we should not skip them when inserting the new decl if the lookup
found nothing.

Fixes PR15841.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180571 91177308-0d34-0410-b5e6-96231b3b80d8