[🍒 stable/20240723] [lldb] Improve editline completion formatting (#116456)

[JDevlieghere]

This patch improves the formatting of editline completions. The current implementation is naive and doesn't account for the terminal width.

Concretely, the old implementation suffered from the following issues:

• We would unconditionally pad to the longest completion. If that completion exceeds the width of the terminal, that would result in a lot of superfluous white space and line wrapping.
• When printing the description, we wouldn't account for the presence of newlines, and they would continue without leading padding.

The new code accounts for both. If the completion exceeds the available terminal width, we show what fits on the current lined followed by ellipsis. We also no longer pad beyond the length of the current line. Finally, we print the description line by line, with the proper leading padding. If a line of the description exceeds the available terminal width, we print ellipsis and won't print the next line.

Before:

Available completions:
        _regexp-attach    -- Attach to process by ID or name.
        _regexp-break     -- Set a breakpoint using one of several shorthand
 formats.
        _regexp-bt        -- Show backtrace of the current thread's call sta
ck. Any numeric argument displays at most that many frames. The argument 'al
l' displays all threads. Use 'settings set frame-format' to customize the pr
inting of individual frames and 'settings set thread-format' to customize th
e thread header. Frame recognizers may filter thelist. Use 'thread backtrace
 -u (--unfiltered)' to see them all.
        _regexp-display   -- Evaluate an expression at every stop (see 'help
 target stop-hook'.)

After:

 Available completions:
        _regexp-attach    -- Attach to process by ID or name.
        _regexp-break     -- Set a breakpoint using one of several shorth...
        _regexp-bt        -- Show backtrace of the current thread's call ...
        _regexp-display   -- Evaluate an expression at every stop (see 'h...

rdar://135818198 (cherry picked from commit dd78d7c7be5b8948cf5841e8033e59adebf230ad)

[JDevlieghere]

@swift-ci test

[JDevlieghere]

@swift-ci test