GetPinnableReference on String.cs (dotnet/coreclr#23428)
GetPinnableReference on String.cs (dotnet/coreclr#23428)

* GetPinnableReference on String.cs

* Add attributes for debugger and performance

Signed-off-by: dotnet-bot <03c4cb6107ae99956a7681f2c97ff7104809a2bd@microsoft.com>

wtgodbe/corefx,wtgodbe/corefx,BrennanConroy/corefx,ViktorHofer/corefx,ericstj/corefx,wtgodbe/corefx,shimingsg/corefx,ViktorHofer/corefx,shimingsg/corefx,ViktorHofer/corefx,shimingsg/corefx,ericstj/corefx,shimingsg/corefx,ericstj/corefx,ViktorHofer/corefx,ViktorHofer/corefx,ericstj/corefx,wtgodbe/corefx,ViktorHofer/corefx,wtgodbe/corefx,ViktorHofer/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,BrennanConroy/corefx,shimingsg/corefx,ericstj/corefx,BrennanConroy/corefx,shimingsg/corefx,wtgodbe/corefx,wtgodbe/corefx