Fix S2386 & S3887 FP: should not be raised for FrozenDictionary and FrozenSet

[T-Emile]

Description

S2386 and S3887 should not be raised for types FrozenDictionary and FrozenSet.

Repro steps

public class Test
{
    public static readonly FrozenDictionary<string, string> FrozenDictionaryTest = new Dictionary<string, string>().ToFrozenDictionary();
    public static readonly FrozenSet<string> FrozenSetTest = new HashSet<string>().ToFrozenSet();
}

Expected behavior

No warnings should be raised.

Actual behavior

Two warnings are raised for both fields:

S2386: Use an immutable collection or reduce the accessibility of the public static field 'FrozenDictionaryTest'.
S3887: Use an immutable collection or reduce the accessibility of the non-private readonly field 'FrozenDictionaryTest'.

[martin-strecker-sonarsource]

Hello @T-Emile
Thank you for reporting this FP. We have added a reproducer and a fix in #8652.