Fix IDE0250 (struct can be made readonly)

Author: stephentoub

src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs

@@ -211,7 +211,7 @@ void ICollection.CopyTo(Array array, int index)
 
         object ICollection.SyncRoot => (m_dictionary is ICollection coll) ? coll.SyncRoot : this;
 
-        private struct DictionaryEnumerator : IDictionaryEnumerator
+        private readonly struct DictionaryEnumerator : IDictionaryEnumerator
         {
             private readonly IDictionary<TKey, TValue> _dictionary;
             private readonly IEnumerator<KeyValuePair<TKey, TValue>> _enumerator;

src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventPipe.cs

@@ -29,7 +29,7 @@ internal struct EventPipeSessionInfo
     }
 
     [StructLayout(LayoutKind.Sequential)]
-    internal struct EventPipeProviderConfiguration
+    internal readonly struct EventPipeProviderConfiguration
     {
         [MarshalAs(UnmanagedType.LPWStr)]
         private readonly string m_providerName;

src/libraries/System.Private.CoreLib/src/System/Threading/ReaderWriterLockSlim.cs

@@ -230,7 +230,7 @@ public void EnterReadLock()
         //
         // Common timeout support
         //
-        private struct TimeoutTracker
+        private readonly struct TimeoutTracker
         {
             private readonly int _total;
             private readonly int _start;