Fix incorrect call to ExprResult::get()

Res is already a ExprResult, so if we call .get(), we will convert an
ExprError() result into an unset result. I discovered this in our downstream
CHERI target where this resulted in a crash due to a NULL-dereference.
It appears that this was introduced in SVN revision 201788 (8690a686)

Reviewed By: Anastasia
Differential Revision: https://reviews.llvm.org/D81608