move AstGen to std.zig.AstGen

Part of an effort to ship more of the compiler in source form.

src/Module.zig

@@ -28,7 +28,7 @@ const link = @import("link.zig");
 const Air = @import("Air.zig");
 const Zir = std.zig.Zir;
 const trace = @import("tracy.zig").trace;
-const AstGen = @import("AstGen.zig");
+const AstGen = std.zig.AstGen;
 const Sema = @import("Sema.zig");
 const target_util = @import("target.zig");
 const build_options = @import("build_options");