Wrapper for LLVM assertion failures

[catamorphism]

It would be great if rustc had a way to print out its own error message on an LLVM assertion failure, along the lines of "this is a bug in the compiler, please report it". Otherwise users may not realize that they're seeing a compiler bug rather than an error in their program.

[bblum]

This would be nonstraightforward: without knowing any LLVM specifics, I'd guess it uses a C assert implementation, which raises SIGABRT. Encapsulating every single call to libllvm that rustc makes would be a huge perf problem, so I expect the best way would be to hack LLVM itself.

[catamorphism]

I no longer think this is very important, as we see LLVM assertion failures much less often than we used to. Closing.