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);
```

1 files changed, 2 insertions, 2 deletions

diff --git a/src/codegen/spirv.zig b/src/codegen/spirv.zig
index 0dc39db134..9879bc7f35 100644
--- a/src/codegen/spirv.zig
+++ b/src/codegen/spirv.zig
@@ -184,7 +184,7 @@ pub const DeclGen = struct {
 
     fn fail(self: *DeclGen, comptime format: []const u8, args: anytype) Error {
         @setCold(true);
-        const src: LazySrcLoc = .{ .node_offset = 0 };
+        const src = LazySrcLoc.nodeOffset(0);
         const src_loc = src.toSrcLoc(self.decl);
         assert(self.error_msg == null);
         self.error_msg = try Module.ErrorMsg.create(self.module.gpa, src_loc, format, args);
@@ -193,7 +193,7 @@ pub const DeclGen = struct {
 
     fn todo(self: *DeclGen, comptime format: []const u8, args: anytype) Error {
         @setCold(true);
-        const src: LazySrcLoc = .{ .node_offset = 0 };
+        const src = LazySrcLoc.nodeOffset(0);
         const src_loc = src.toSrcLoc(self.decl);
         assert(self.error_msg == null);
         self.error_msg = try Module.ErrorMsg.create(self.module.gpa, src_loc, "TODO (SPIR-V): " ++ format, args);