Change StreamReader.ReadBufferAsync to return ValueTask<int> (dotnet/coreclr#24810)

It's a private helper that's always directly awaited.  No need to allocate a task each time it's called and data is synchronously available.

Commit migrated from https://github.com/dotnet/coreclr/commit/13405da4ba21095f7673a10eaabfef990117478d

src/libraries/System.Private.CoreLib/src/System/IO/StreamReader.cs

@@ -1238,7 +1238,7 @@ public override ValueTask<int> ReadBlockAsync(Memory<char> buffer, CancellationT
             return new ValueTask<int>(t);
         }
 
-        private async Task<int> ReadBufferAsync()
+        private async ValueTask<int> ReadBufferAsync()
         {
             _charLen = 0;
             _charPos = 0;