Move String.Manipulation.cs to shared (dotnet/coreclr#16226)

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: jkotas

src/Common/src/CoreLib/System.Private.CoreLib.Shared.projitems

@@ -487,6 +487,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Span.Fast.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\SpanDebugView.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Span.NonGeneric.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\String.Manipulation.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\String.Searching.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\StringSpanHelpers.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\StackOverflowException.cs" />

src/Common/src/CoreLib/System/Char.cs

@@ -401,14 +401,14 @@ public static char ToUpper(char c, CultureInfo culture)
         //
         public static char ToUpper(char c)
         {
-            return ToUpper(c, CultureInfo.CurrentCulture);
+            return CultureInfo.CurrentCulture.TextInfo.ToUpper(c);
         }
 
 
         // Converts a character to upper-case for invariant culture.
         public static char ToUpperInvariant(char c)
         {
-            return ToUpper(c, CultureInfo.InvariantCulture);
+            return CultureInfo.InvariantCulture.TextInfo.ToUpper(c);
         }
 
 
@@ -432,14 +432,14 @@ public static char ToLower(char c, CultureInfo culture)
         // Converts a character to lower-case for the default culture.
         public static char ToLower(char c)
         {
-            return ToLower(c, CultureInfo.CurrentCulture);
+            return CultureInfo.CurrentCulture.TextInfo.ToLower(c);
         }
 
 
         // Converts a character to lower-case for invariant culture.
         public static char ToLowerInvariant(char c)
         {
-            return ToLower(c, CultureInfo.InvariantCulture);
+            return CultureInfo.InvariantCulture.TextInfo.ToLower(c);
         }