Prevent SVD2LLDB plugin init symbol from being stripped in release builds

[iCMDdev]

In SVD2LLDB release builds, there is no symbol for LLDB's entry point. Thus, loading the plugin fails with the following error:

(lldb) plugin load .build/release/libSVD2LLDB.dylib
error: plug-in is missing the required initialization: lldb::PluginInitialize(lldb::SBDebugger)

This PR adds @_used and public to the LLDB plugin entrypoint pluginInitialize (lldb::PluginInitialize(lldb::SBDebugger)), so the function is retained and exported for release builds as well.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary (no changes required)

[iCMDdev]

Note: it seems to work without @_used as well.

[iCMDdev]

Thanks for the workflow run! Seems that @_used is still experimental and requires a special flag:

51 | @_used
   | `- error: attribute requires '-enable-experimental-feature SymbolLinkageMarkers'

Since everything seems to work without it, perhaps it's better to remove it.