DeferredDiagnosticsEmitter crashes

Patch VisitCXXDeleteExpr() in clang::UsedDeclVisitor to avoid it crashing
when the expression's destroyed type is null. According to the comments
in CXXDeleteExpr::getDestroyedType(), this can happen when the type to
delete is a dependent type.

Patch by Geoff Levner.

Differential Revision: https://reviews.llvm.org/D88949