Part three of PR15721: if we have an invalid CXXDefaultInitExpr, don't crash if
we try to constant-evaluate it. Patch by Karthik Bhat, test by me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190722 91177308-0d34-0410-b5e6-96231b3b80d8