len() of a constant string16 is incorrect

```
Odin: dev-2026-02:6386b395d
OS: Windows 10 Professional (version: 22H2), build 19045.6466
CPU: Intel(R) Core(TM) i5-9600KF CPU @ 3.70GHz
RAM: 16322 MiB
Backend: LLVM 20.1.0
```
```odin
package main

import "core:fmt"

main :: proc() {
	const :: string16("😟")
	var := string16("😟")
	fmt.printfln("%v %v", len(const), len(var)) 
	// expected: 2 2
	// got: 4 2
}
```