genmc: Build without LLVM; adjust to API changes

[michaliskok]

This PR bumps the GenMC version to 0.15.1:

• LLVM is no longer required to build the GenMC library. This results to faster build times and likely unblocks Enable MacOS support for GenMC mode. #4571
• Invalid configs do not cause GenMC to panic now. Instead, GenMC returns a list of warnings/errors that Miri can use as it sees fit. (The code currently just aborts.)
• Miri may now pass debugging information to GenMC so that better error reports are produced. (Currently unused.)
• GenMC's scheduler returns the reason a given execution was dropped, so no heuristics have to be used in Miri.

r? @RalfJung
(Each commit can be reviewed separately.)

[RalfJung]

Looks like CI has some trouble:

    /home/runner/work/miri/miri/genmc-sys/genmc-src/src/ExecutionGraph/ExecutionGraph.cpp: In member function 'void ExecutionGraph::validate()':
    /home/runner/work/miri/miri/genmc-sys/genmc-src/src/ExecutionGraph/ExecutionGraph.cpp:496:38: error: 'print' is not a member of 'std'; did you mean 'printf'?
      496 |                                 std::print(std::cerr, "Non-existent RF: {}\n{}\n", rLab->getPos(),
          |                                      ^~~~~
          |                                      printf

CI should be using g++ 13.2. Is that not new enough for std::print...?

[RalfJung]

Yeah apparently that needs GCC 14.1. I don't know whether there's a way to get that on Github's CI. But even if there is, if not even Ubuntu 24 has a new enough GCC I'm quite concerned that this might just be too recent. New GCC versions tend to spread very slowly and we can't make it too hard for people to build Miri.

[michaliskok]

Yeah apparently that needs GCC 14.1. I don't know whether there's a way to get that on Github's CI. But even if there is, if not even Ubuntu 24 has a new enough GCC I'm quite concerned that this might just be too recent. New GCC versions tend to spread very slowly and we can't make it too hard for people to build Miri.

I'm a bit surprised that cmake configure didn't fail. But anyway I'll ensure it builds with 13.2.

[michaliskok]

@rustbot ready

[RalfJung]

Looks great, thanks!