[lldb] Fix term-width setting (#82736)

I noticed that the term-width setting would always report its default
value (80) despite the driver correctly setting the value with
SBDebugger::SetTerminalWidth.

```
(lldb) settings show term-width
term-width (int) = 80
```

The issue is that the setting was defined as a SInt64 instead of a
UInt64 while the getter returned an unsigned value. There's no reason
the terminal width should be a signed value. My best guess it that it
was using SInt64 because UInt64 didn't support min and max values. I
fixed that and correct the type and now lldb reports the correct
terminal width:

```
(lldb) settings show term-width
term-width (unsigned) = 189
```

rdar://123488999