Fix the AST printer for attributed statements so that it does not print...

Fix the AST printer for attributed statements so that it does not print duplicate attribute introducers. Eg) [[clang::fallthrough]] instead of  [[[[clang::fallthrough]]]]

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208706 91177308-0d34-0410-b5e6-96231b3b80d8