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
Move function emitDeferredDiags from Sema to DeferredDiagsEmitter since it
is only used by DeferredDiagsEmitter.
Also skip visited functions to avoid exponential compile time.
Differential Revision: https://reviews.llvm.org/D77028
Erich Keane