[lldb] Add setting for max depth of value object printing (NFC)

[lldb] Add setting for max depth of value object printing (NFC)

This adds a setting (`target.max-children-depth`) that will provide a default value for the `--depth` flag used by `expression` and `frame variable`.

The new setting uses the same default that's currently fixed in source: `UINT32_MAX`.

This provides two purposes:

1. Allowing downstream forks to provide a customized default.
2. Allowing users to set their own default.

Following `target.max-children-count`, a warning is emitted when the max depth is reached. The warning lets users know which flags or settings they can customize. This warning is shown only when the limit is the default value.

rdar://87466495

Differential Revision: https://reviews.llvm.org/D123954