If definition of default function argument uses itself, clang crashed,
because corresponding function parameter is not associated with the default
argument yet. With this fix clang emits appropriate error message.

This change fixes PR28105.

Differential Revision: http://reviews.llvm.org/D21301


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272623 91177308-0d34-0410-b5e6-96231b3b80d8