Suppress Deferred Diagnostics in discarded statements.

It doesn't really make sense to emit language specific diagnostics
in a discarded statement, and suppressing these diagnostics results in a
programming pattern that many users will feel is quite useful.

Basically, this makes sure we only emit errors from the 'true' side of a
'constexpr if'.

It does this by making the ExprEvaluatorBase type have an opt-in option
as to whether it should visit discarded cases.

Differential Revision: https://reviews.llvm.org/D102251