Avoid crash if default argument parsed with errors.

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

llvm-svn: 213594