RuntimeResourceSet improvements (#44454)

* RuntimeResourceSet improvements

- remove unnecessary base class calls to trim more of base type
- remove code paths which were never executed
- replaced nested locking with simple locks
- fix caching for case insensitive mode
- add tests for more code paths

* Move local caseInsensitiveTable initialisation

* Clean up unused code

* Revert "Clean up unused code"

It's actually used via reflection in System.Resources.Extensions

src/libraries/System.Private.CoreLib/src/System/Resources/ResourceReader.cs

@@ -27,26 +27,16 @@ namespace System.Resources
     // default file format.
     //
 
-    internal struct ResourceLocator
+    internal readonly struct ResourceLocator
     {
-        internal object? _value;  // Can be null.
-        internal int _dataPos;
-
         internal ResourceLocator(int dataPos, object? value)
         {
-            _dataPos = dataPos;
-            _value = value;
+            DataPosition = dataPos;
+            Value = value;
         }
 
-        internal int DataPosition => _dataPos;
-
-        // Allows adding in profiling data in a future version, or a special
-        // resource profiling build.  We could also use WeakReference.
-        internal object? Value
-        {
-            get => _value;
-            set => _value = value;
-        }
+        internal int DataPosition { get; }
+        internal object? Value { get; }
 
         internal static bool CanCache(ResourceTypeCode value)
         {
@@ -800,8 +790,12 @@ private void _ReadResources()
             // Read RuntimeResourceSet header
             // Do file version check
             int version = _store.ReadInt32();
-            if (version != RuntimeResourceSet.Version && version != 1)
-                throw new ArgumentException(SR.Format(SR.Arg_ResourceFileUnsupportedVersion, RuntimeResourceSet.Version, version));
+
+            // File format version number
+            const int CurrentVersion = 2;
+
+            if (version != CurrentVersion && version != 1)
+                throw new ArgumentException(SR.Format(SR.Arg_ResourceFileUnsupportedVersion, CurrentVersion, version));
             _version = version;
 
             _numResources = _store.ReadInt32();