Cannot start a program after a successful build: 'Illegal instruction (core dumped)'


I have been using LLVM to build ungoogled-chromium (UC)
since Sept.2018. I have also been updating LLVM from
SVN and rebuilding it without any issues.

The other day I updated and rebuilt LLVM again after
which I rebuilt UC - the same version which has been
running fine to that moment. However when trying to
run the program I got 'Illegal instruction (core
dumped)' and UC simply didn't start. That was a build
made using clang version 8.0.0 (trunk 346584).

I had no idea what was causing it but I found an older
version of LLVM on my disk, so I decided to rebuild UC
using it. So using clang version 8.0.0 (trunk 346299)
the build was successful and UC works just fine.

Here are more details on the issue:

Is this a bug in LLVM? If yes - how to report it
(considering I have just 'Illegal instruction (core
dumped)') as a feedback. If no - what is the solution
to this?