src/cpp-common: add bt2c::textLocStr() function

src/cpp-common: add bt2c::textLocStr() function

This new function formats a text location (`bt2c::TextLoc`) according
to some format amongst:

`bt2c::TextLocStrFmt::Offset`:
    Offset only.

`bt2c::TextLocStrFmt::LineColNosAndOffset`:
    Line/column numbers and offset.

`bt2c::TextLocStrFmt::LineColNos`:
    Line/column numbers only.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I8afeb43a71c7103135f903f7d4eeb73ccf9e24c3
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12679