[flang] Do not stop on mismatched DATA substring length (#69336)

https://reviews.llvm.org/D143819 turned mismatched DATA substring from a
crash into a warning. However, the resulting DATA was incorrect when
there were subsequent DATA values after the mismatch because the DATA to
init conversion stopped. This change let the DATA to init continue.

I added a LengthMismatch tag instead of using SizeMismatch because the
other situation where SizeMismatch is returned seem like bug situations
to me (the DATA value is dropped and the offset is not advanced), so I
did not want to continue DATA processing in these cases.