The CMemoryReader interface relies on a `GetStringLength` callback, which
returns zero either if the address is invalid or if a valid zero-length
string exists at the given address. We don't want to break ABI with
RemoteMirror, but we can work around this by issuing a one-byte read
at the address and confirming that a null terminator exists there.
* Change the RemoteMirror API to have extensible data layout callback
* Use DLQ_Get prefix on DataLayoutQueryType enum values
* Simplify MemoryReaderImpl and synthesize minimalDataLayoutQueryFunction
This tool should test the usage from SwiftRemoteMirror dylib and
the C API, since that is the public interface from which we're
vending the remote reflection functionality.
Memory readers on the C-side of the API may actually have an object-
oriented design, so they may want to pass an instance to revive
when callbacks make it to the other side of the API boundary.
