Make the error condition in Value::ValueType explicit (NFC)

The comment for ValueType claims that all values <1 are errors, but
not all switch statements take this into account. This patch
introduces an explicit Error case and deletes all default: cases, so
we get warned about incomplete switch coverage.

https://reviews.llvm.org/D96537

GitOrigin-RevId: 057efa9916cdc354ef4653bcd128a578cc43125e
35 files changed