LLDB has the concept of an "SDK root", which expresses where the
system libraries and so which are necessary for debugging can be
found. On POSIX platforms, the SDK root is just the root of the
file system, or /.
We need to implement the appropriate method on HostInfoPosix to
prevent the use of the default implementation for which just returns
an error.
This change is needed to support the Swift REPL but may be useful
for other use cases as well.
0b98b27930