CA1859 false positive when a file-local type is involved

### Describe the bug
CA1859 when a file-local type is returned from a method. Doing what it suggests would break the build with
`CS9051: File-local type 'Bar' cannot be used in a member signature in non-file-local type 'C'.`

### To Reproduce
```cs
class C
{
// CA1859: Change return type of method 'Foo' from 'System.IDisposable' to 'Bar' for improved performance
IDisposable Foo() => new Bar();
}

file class Bar : IDisposable
{
public void Dispose() {}
}
```

### Exceptions (if any)

### Further technical details
details of dotnet --info

.NET SDK:
Version: 10.0.300
Commit: caa81fa497
Workload version: 10.0.300-manifests.e0989437
MSBuild version: 18.6.3+caa81fa49

Runtime Environment:
OS Name: Windows
OS Version: 10.0.26200
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\10.0.300\

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
Version: 10.0.8
Architecture: x64
Commit: 94ea82652c

.NET SDKs installed:
6.0.428 [C:\Program Files\dotnet\sdk]
10.0.106 [C:\Program Files\dotnet\sdk]
10.0.300 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.15 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
DOTNET_CLI_TELEMETRY_OPTOUT [1]

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

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- The IDE (VS / VS Code/ VS4Mac) you're running on, and its version

VS 18.6.0