-preview=in deprecation warning

[Jack Applegame]

Can anyone help me get rid of this depreciation?

```d
struct Foo {
	string foo;
	string getFoo() const @safe { return foo; }
}

size_t bar(in Foo foo) @safe {
	return foo.getFoo().length;
}

void main() {
	Foo("hello").bar().writeln();
}
```
```sh
$ ldc2 -preview=in --run foo.d
foo.d(9): Deprecation: scope variable `foo` assigned to non-scope 
parameter `this` calling `getFoo`
```
```sh
$ ldc2 --version
LDC - the LLVM D compiler (1.32.1):
   based on DMD v2.102.2 and LLVM 15.0.7
```