Currently the fix hint is hardcoded to gsl::at(). This poses
a problem for people who, for a number of reasons, don't want
or cannot use the GSL library.
In these situations, the fix hint does more harm than good
as it creates confusion as to what the fix should be. People
can even misinterpret the fix "gsl::at" as e.g. "std::array::at",
which can lead to even more trouble (e.g. when having guidelines
that disallow exceptions).
Therefore, introduce a new option FixHint that allows users
to specify exactly what the fix hint should be for their
use case. The default is kept to gsl::at.
Since the users can write anything in this option, clang-tidy
should not add fix-it hints in this case, as they could
lead to invalid code. For example, a reasonable fix hint
like "MyCustomSafeArrayWrapper::at()" would not be possible
to replace directly in code, yet it would be of great help
pointing to users in the right direction.