[flang] Fix assert on bad character kind

When we report an error for a bad character kind, don't keep it in the
`DeclTypeSpec`. Otherwise there could be further problems. In this case,
`ComputeOffsets()` got an assertion error because we didn't recognize
`CHARACTER(*,8)` as needing a descriptor because of the bad kind.

Fixes https://bugs.llvm.org/show_bug.cgi?id=47173

Differential Revision: https://reviews.llvm.org/D86357