[ASTImporter] FriendDecl importing improvements

There are only a few cases of importing a frienddecl which is currently supported.
This patch aims to improve the friend import process.
Set FriendObjectKind in case of decls, insert friend into the friend chain
correctly, checks structurally equivalent in a more advanced manner.
Test cases added as well.  



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@330847 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed