Avoid crash if default argument parsed with errors.

If function parameters have default values, and that of the second
parameter is parsed with errors, function declaration would have
a parameter without default value that follows a parameter with
that. Such declaration breaks logic of selecting overloaded
function. As a solution, put opaque object as default value in such case.

This patch fixes PR20055.

Differential Revision: http://reviews.llvm.org/D4378


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213594 91177308-0d34-0410-b5e6-96231b3b80d8