Fix an edge case in the template differ with default arguments.

In the test case one type is coming from a typedef with no default arg, the
other has the default arg. Taking the default arg from the typedef crashes, so
always use the real template paramter declaration. PR17510.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192202 91177308-0d34-0410-b5e6-96231b3b80d8