Change FieldSensitive's enum representation to allow distinguishing
among the elements with associated value. Consider
`unchecked_take_enum_data_addr` to consume all other fields than that
taken.
rdar://125113258
The visitor returns false to indicate an unknown/unhandled instruction.
Previously it returned true as a fallback. This resulted in aberrant
behavior: e.g., in the test case committed here, the try_apply was
deleted.
