Handle FriendDecl in DeclContextPrinter
This commit fixes a crash that occurs when -print-decl-contexts AST consumer
tries to print an unhandled declaration.
rdar://19467234
Differential Revision: https://reviews.llvm.org/D26964
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290880 91177308-0d34-0410-b5e6-96231b3b80d8