[Sema] Avoid isNullPointerConstant invocation

DiagnoseNullConversion is needlessly calling isNullPointerConstant which
is an expensive routine due to its calls to a constant evaluator --
which we don't need.

Building the Linux Kernel (x86_64) with this fix has improved build
times by ~2.1%. This is mainly due to the following methods no longer
needing to be called anywhere near as often:
1) ExprConstant::CheckICE (reduced CPU cycles by ~90%)
2) IntExprEvaluator::VisitBinaryOperator (reduced CPU cycles by ~50%)

Reviewed By: rtrieu, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D131532