Add pass to turn function into Unreachable and script to find UB. (WIP)

[fhahn]

Summary

This adds a pass to replace function bodies with 'unreachable' and a
script that checks if the result still verifies.

This can be used to find functions in llvm-lit tests that have
unconditional UB using

llvm-lit -j4 -sv -Dopt=~/find-ub-in-test.sh

Examples of found & fixed UB:

• b8d728a098b1
• 684a82fbc543