[lldb] Improve ansi::OutputWordWrappedLines (#181165)
This PR fixes a few issues including one that prevented the use of
llvm::StringRef. Follow up to #180947.
Some behaviour for rarely seen inputs has been defined. For example
empty strings. In normal use with command descriptions we do not expect
this to happen, but now it's a utility function, it's easier to reason
about if we cover all possible inputs.
* Empty string in now results in an empty string out. Rather than a
single newline. This is less surprising, since no lines were split.
* Bugs were fixed in the handling of single word inputs. If a single
word cannot fit within the column limit we just print it unmodified.
* Leading spaces are trimmed from input and if that results in no text,
empty string is returned. Another unexpected input, but cheap to handle
and makes the rest of the code a bit simpler.
* llvm::StringRef is now used for the input text. This was enabled by
fixing a bug in checking whether end had reached final_end. I think the
previous logic caused us to read out of bounds by 1 byte sometimes.
* Some buggy test output has changed but this is fine, no one is relying
on that buggy behaviour.
Ultimately I may wipe out all these changes later with a new
implementaion, but I think it's a good approach to do this
incremenetally at first.
