Investigate error message from `CheriNeedBoundsChecker::useNeedsBounds`

[xdoardo]

Which under some circumstances falls in the default case, which prints a generic error message without actually erroring out.

llvm-project/llvm/lib/Analysis/CheriBounds.cpp

Line 240 in 928dad3

errs() << "DON'T know how to handle ";

[resistor]

#287