Fix the const overload of IgnoreExprNodes

The existing code causes an infinite recursion. This is likely an
oversight of commit c0e5e3fb [1] where the intent must have been
to call the non-const overload.

[1] https://reviews.llvm.org/D87278

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D100347