[clang] Do not crash after suggesting typo correction to constexpr if condition

In some cases non-null non-constant yet valid expression may reach point where
`ConditionResult` is created. For example, typo correction mechanism can return
such expression, so double check before evaluating it.

Fixes https://github.com/llvm/llvm-project/issues/61885

Reviewed By: tbaeder, aaron.ballman

Differential Revision: https://reviews.llvm.org/D148206