Figure out missing source location for vtable initializers.

[vecchiot-aws]

[vecchiot-aws]

https://github.com/vecchiot-aws/rmc/tree/vtable-initialize-location

[vecchiot-aws]

Currently blocked; the source locations correctly appear in the XML output, but do not appear in the HTML output.

[tautschnig]

Closing as there isn't enough information to see where we still might be missing locations (#3253 does add a locations to some vtable-related expressions).