[lldb/StringPrinter] Support strings with invalid utf8 sub-sequences

Support printing strings which contain invalid utf8 sub-sequences, e.g.
strings like "hello world \xfe", instead of bailing out with "Summary
Unavailable".

I took the opportunity here to delete some hand-rolled utf8 -> utf32
conversion code and replace it with calls into llvm's Support library.

rdar://61554346