[LSP] Microsoft.CodeAnalysis.LanguageServer shows high CPU usage when performing `switch` on a `union` in C# 15.

**Version Used**:

- .NET 11 Preview 4 (11.0.100-preview.4.26230.115)
- VSCode
- C# 2.136.19
- C# Dev Kit 3.14.196

**Steps to Reproduce:**

Creating C# 15 `union` with several dozen elements and performing pattern matching with a switch statement will make the language server extremely slow. (The CPU usage of the Microsoft.CodeAnalysis.LanguageServer process becomes extremely high.)

```cs
Alphabet a = new A();
switch (a)
{
case A:
case B:
...
}

union Alphabet(A, B, C, ..., Z);
```

This issue does not occur if you replace it with a switch statement using `object? Value`.

```cs
Alphabet a = new A();
switch (a.Value)
{
case A:
case B:
...
}

union Alphabet(A, B, C, ..., Z);
```

Reproduce the issue in VSCode using .NET 11 Preview 4, C# 2.136.19, and C# Dev Kit 3.14.196 with a several-dozen-element C# 15 union. Compare pattern matching on the union with matching on object? Value; done means the Microsoft.CodeAnalysis.LanguageServer no longer shows extremely high CPU or becomes very slow.

Written by the indexing model from the issue text.