Rename system plugin directory from /usr/lib/lldb to /usr/lib/lldb/plugins

[k8stone]

Summary

Trivial change to create a dedicated directory for plugins. Recursively scanning /usr/lib/lldb alone has made it far too easy to add files that are dynamically loaded without meaning to (notably ASAN support libraries that then terminate the LLDB process immediately because the ASAN runtime isn't present.)

[labath]

We are not depending on this directory (in fact, this is the first time I have seen it exists :) ). The new directory name sounds reasonable.

[k8stone]

Presumably we've had enough visibility here to proceed. Thanks!

source/Host/linux/HostInfoLinux.cpp

 bool
 HostInfoLinux::ComputeSystemPluginsDirectory(FileSpec &file_spec)
 {
-    FileSpec temp_file("/usr/lib/lldb", true);
+    FileSpec temp_file("/usr/lib/lldb/plugins", true);
     file_spec.GetDirectory().SetCString(temp_file.GetPath().c_str());
     return true;
 }