Fixed keyword triggers compiler error in implicit entrypoint

Using the `fixed` keyword in the implicit entrypoint triggers a compiler error. The same code in a function compiles just fine.

**Repro steps**

Provide the steps required to reproduce the problem:

1. Create new .NET 6 F# console app
2. Use the following code:
```fsharp
let inFunctionScope () =
let arr = [|1; 2; 3|]
use arrFixed = fixed arr // Works
()

let arr2 = [|1; 2; 3|]
use arrFixed2 = fixed arr2 // Compiler error

()
```

Repro ZIP file:
[FsharpBugFixedKeyword.zip](https://github.com/dotnet/fsharp/files/7781720/FsharpBugFixedKeyword.zip)

**Expected behavior**

No compiler error

**Actual behavior**

Compiler error:
```
Program.fs(7,5): error FS3207: Invalid use of 'fixed'. 'fixed' may only be used in a declaration of the form 'use x = fixed expr' where the expression is an array, the address of a field, the address of an array element or a string'
```

**Known workarounds**

Put fixed keyword into a function

**Related information**

* Operating system: Win11
* .NET Runtime kind (.NET Core, .NET Framework, Mono): .NET 6 SDK (6.0.100)
* Editing Tools (e.g. Visual Studio Version, Visual Studio): Rider (reproduced from command line too)