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/c.zig

@@ -363,7 +363,7 @@ pub const DeclGen = struct {
 
     fn fail(dg: *DeclGen, comptime format: []const u8, args: anytype) error{ AnalysisFail, OutOfMemory } {
         @setCold(true);
-        const src: LazySrcLoc = .{ .node_offset = 0 };
+        const src = LazySrcLoc.nodeOffset(0);
         const src_loc = src.toSrcLoc(dg.decl);
         dg.error_msg = try Module.ErrorMsg.create(dg.module.gpa, src_loc, format, args);
         return error.AnalysisFail;