Serialization: Error when decoding invalid PlatformKind values.
Recently, unexpected binary module sharing between compilers that had
inconsistent views of the `PlatformKind` enumeration caused me to spend several
days debugging undefined behavior in the compiler. We should validate that
enumeration values decoded during deserialization are valid, and fail fast when
they are not to make debugging this kind of issue less time consuming. This
change just validates the `PlatformKind` value decoded for an `AvailableAttr`,
but more of deserialization could be updated to do similar validation.
Resolves rdar://123770273
