stage2: implement caching for ZIR code

Notably this exposed an issue with the language having to do with the
secret safety tag on untagged unions. How can we have our cake and eat
it too? Not solved in this commit. I will file a language proposal to
tackle this issue soon.

Fixes a compile error in `std.fs.File.readvAll`.

src/main.zig

@@ -3606,7 +3606,7 @@ pub fn cmdAstgen(
 
     if (file.zir.hasCompileErrors()) {
         var errors = std.ArrayList(Compilation.AllErrors.Message).init(arena);
-        try Compilation.AllErrors.addZir(arena, &errors, &file, source);
+        try Compilation.AllErrors.addZir(arena, &errors, &file);
         const ttyconf = std.debug.detectTTYConfig();
         for (errors.items) |full_err_msg| {
             full_err_msg.renderToStdErr(ttyconf);