Replace custom AnsiToLower with more efficient ToLowerAsciiInvariant (#21121)

Avoids all of the StringBuilder-related costs.

Author: stephentoub

src/System.Private.CoreLib/shared/System/Globalization/CultureData.cs

@@ -2407,35 +2407,8 @@ internal void GetNFIValues(NumberFormatInfo nfi)
 
         // Helper
         // This is ONLY used for caching names and shouldn't be used for anything else
-        internal static string AnsiToLower(string testString)
-        {
-            int index = 0;
-
-            while (index<testString.Length && (testString[index]<'A' || testString[index]>'Z' ))
-            {
-                index++;
-            }
-            if (index >= testString.Length)
-            {
-                return testString; // we didn't really change the string
-            }
-
-            StringBuilder sb = new StringBuilder(testString.Length);
-            for (int i=0; i<index; i++)
-            {
-                sb.Append(testString[i]);
-            }
-
-            sb.Append((char) (testString[index] -'A' + 'a'));
-
-            for (int ich = index+1; ich < testString.Length; ich++)
-            {
-                char ch = testString[ich];
-                sb.Append(ch <= 'Z' && ch >= 'A' ? (char)(ch - 'A' + 'a') : ch);
-            }
-
-            return (sb.ToString());
-        }
+        internal static string AnsiToLower(string testString) =>
+            TextInfo.ToLowerAsciiInvariant(testString);
 
         /// <remarks>
         /// The numeric values of the enum members match their Win32 counterparts.  The CultureData Win32 PAL implementation

src/System.Private.CoreLib/shared/System/Globalization/TextInfo.cs

@@ -451,7 +451,7 @@ private unsafe string ChangeCaseCommon<TConversion>(string source) where TConver
             }
         }
 
-        private static unsafe string ToLowerAsciiInvariant(string s)
+        internal static unsafe string ToLowerAsciiInvariant(string s)
         {
             if (s.Length == 0)
             {