Explicitly globalize $wpdb in either wp-settings.php or ms-settings.php

[danielbachhuber]

ms-settings.php implicitly expects $wpdb to be a globalized variable. However, neither wp-settings.php nor ms-settings.php explicitly globalizes it, which causes a fatal when wp-settings.php is called within a function.

$wpdb should be explicitly globalized, either in wp-settings.php or ms-settings.php. I don't have a strong preference as to which.

From ​https://github.com/wp-cli/wp-cli/pull/3031/commits/dc38f0834d6cd47f3c424e5ae2d357da81567f54

[SergeyBiryukov]

In 37864:

Database: Explicitly globalize $wpdb in wp-settings.php in case WordPress isn't loaded in global scope.

See [18993] for $table_prefix.

Props danielbachhuber.
Fixes #37123.