ast, hir, llvm: crash debug information

Currently, we have a lot of crashes. Most of these happen in HIR, where most of the logic happens (initial flattening of the AST, followed by a lot of semantic analysis). Ast crashes occasionally, as well as codegen. Debugging these is difficult because the backtrace that zig provides isn't enough to figure out which part of the input source code triggered the issue - so we end up commenting out code until we find the line that crashes. It would be great to have some debug reporting that indicates the current token/statement/instruction being parsed during a crash and where in the pipeline we are.