introduce std.debug.Trace

And use it to debug a LazySrcLoc in stage2 that is set to a bogus value.

The actual fix in this commit is:

```diff
-        try sema.emitBackwardBranch(&child_block, call_src);
+        try sema.emitBackwardBranch(block, call_src);
```

src/codegen/llvm.zig

@@ -2163,7 +2163,7 @@ pub const DeclGen = struct {
     fn todo(self: *DeclGen, comptime format: []const u8, args: anytype) Error {
         @setCold(true);
         assert(self.err_msg == null);
-        const src_loc = @as(LazySrcLoc, .{ .node_offset = 0 }).toSrcLoc(self.decl);
+        const src_loc = LazySrcLoc.nodeOffset(0).toSrcLoc(self.decl);
         self.err_msg = try Module.ErrorMsg.create(self.gpa, src_loc, "TODO (LLVM): " ++ format, args);
         return error.CodegenFail;
     }