swift234

teach FailureDiagnosis::typeCheckChildIndependently not to recurse into
DiscardAssignmentExpr's, because they require context to typecheck anyway
and doing so triggers bogus errors about _ needing to be on the LHS of an
assignment.  There is a correct way to fix this, but layers of issues need
to be peeled off before that can happen.

This fixes <rdar://problem/21883806> Bogus "'_' can only appear in a pattern or on the left side of an assignment" is back

but the new diagnostic that is revealed now that this bogus one is removed is
also really bad.



Swift SVN r30369