The current ProgrammersManual.rst document has a lot of well-written documentation on error handling thanks to @lhames. It suggests errors can be split cleanly into "programmatic" and "recoverable" errors. However, the reality in current LLVM seems to be there are a number of cases where a non-programmatic error is not easily recoverable. Therefore, add a note to indicate the existence of report_fatal_error for these cases. I've also added a reminder to CodingStandards.rst in the section on assertions, to indicate that llvm_unreachable and assertions should not be relied upon to report errors triggered by user input.
Thanks to @majnemer for pointing out out my llvm_unreachable abuse in a number of my RISC-V patches.
The ProgrammersManual is also silent on the use of LLVMContext::diagnose, which is used in BPF+WebAssembly+AMDGPU to report some errors during instruction selection. I don't address that in this patch, as it's not quite clear how to fit in to the current error handling story.