[flang] Recode a switch() to dodge a sketchy warning

One of the buildbots uses a compiler (can't tell which) that
doesn't approve of a "default:" in a switch statement whose
cases appear to completely cover all possible values of an
enum class.  But this switch is in raw data dumping code that
needs to allow for incorrect values in memory.  So rewrite it
as a cascade of if statements; performance doesn't matter here.