Simplify NotNullIfParameterNotNull implementation in PEParameterSymbol

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEParameterSymbol.cs

@@ -706,20 +706,7 @@ internal override ImmutableHashSet<string> NotNullIfParameterNotNull
         {
             get
             {
-                var attributes = GetAttributes();
-                var result = ImmutableHashSet<string>.Empty;
-                foreach (var attribute in attributes)
-                {
-                    if (attribute.IsTargetAttribute(this, AttributeDescription.NotNullIfNotNullAttribute))
-                    {
-                        if (attribute.DecodeNotNullIfNotNullAttribute() is string parameterName)
-                        {
-                            result = result.Add(parameterName);
-                        }
-                    }
-                }
-
-                return result;
+                return _moduleSymbol.Module.GetStringValuesOfNotNullIfNotNullAttribute(_handle);
             }
         }
 

src/Compilers/Core/Portable/MetadataReader/PEModule.cs

@@ -1199,6 +1199,27 @@ internal bool HasMaybeNullWhenOrNotNullWhenOrDoesNotReturnIfAttribute(EntityHand
             return false;
         }
 
+        internal ImmutableHashSet<string> GetStringValuesOfNotNullIfNotNullAttribute(EntityHandle token)
+        {
+            var attributeInfos = FindTargetAttributes(token, AttributeDescription.NotNullIfNotNullAttribute);
+
+            var result = ImmutableHashSet<string>.Empty;
+            if (attributeInfos is null)
+            {
+                return result;
+            }
+
+            foreach (var attributeInfo in attributeInfos)
+            {
+                if (TryExtractStringValueFromAttribute(attributeInfo.Handle, out string parameterName))
+                {
+                    result = result.Add(parameterName);
+                }
+            }
+
+            return result;
+        }
+
         internal CustomAttributeHandle GetAttributeUsageAttributeHandle(EntityHandle token)
         {
             AttributeInfo info = FindTargetAttribute(token, AttributeDescription.AttributeUsageAttribute);