gdb/dap: fix decode_source The documentation for the Source interface says * The path of the source to be shown in the UI. * It is only used to locate and load the content of the source if no * `sourceReference` is specified (or its value is 0). but the code used `path` first. I fixed it to use `sourceReference` first. Approved-By: Tom Tromey <tom@tromey.com>