Stop double-diagnosing explicit convert operator in switch condition (#89142)

Note this also likely fixes a bunch of other cases. We were
double-diagnosting in a template because we were generating the
expression anyway, so any attempts to instantiate the function would
instantiate the expression, thus re-diagnosing it.

This patch replaces it with a RecoveryExpr. Additionally,
VerifyIntegerConstantExpression couldn't handle the RecoveryExpr, as it
requires a non-dependent expression result (which a RecoveryExpr is
dependent). Additionally, callers of it use the return value to decide
that VerifyIntegerConstantExpression succeeded, so it fails if it sees a
RecoveryExpr.