Make `DeviceType` constructor not throw. (#9714)
This PR refactors `DeviceType` so that it won't throw an exception in
any case, e.g.: (a) when `XlaDeviceTypeToString` is called with
`PLUGIN`; or (b) when `XlaDeviceTypeToString` is called with some
unspecified value (i.e. arbitrary integer is cast to `XlaDeviceType`.
**Key Changes:**
- Create `kNativeXlaDeviceTypeNames` array for associating each
`XlaDeviceType` with its name
- For each element at position `i` in the array,
`kNativeXlaDeviceTypeNames[i]` corresponds to the name of whatever
`XlaDeviceType` integer representation is `i` (i.e.
`static_cast<XlaDeviceType>(i)`)
- Refactor `NativeXlaDeviceTypeToString` and `StringToXlaDeviceType`
- Make them into simple static functions
- Adapt them so as to use `kNativeXlaDeviceTypeNames`
- Make `NativeXlaDeviceTypeToString` crash if a non-native (e.g.
`PLUGIN`) `XlaDeviceType` is passed as argument
