Make GetReverseIndex Experimental again (dotnet#13788)

Author: KevinRansom

src/FSharp.Core/prim-types.fs

@@ -4107,6 +4107,7 @@ namespace Microsoft.FSharp.Collections
                 let start = if i < 0 then 0 else i
                 PrivateListHelpers.sliceTake (j - start) (PrivateListHelpers.sliceSkip start l)
 
+        [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
         member l.GetReverseIndex(_: int, offset: int) =
             l.Length - offset - 1
 
@@ -6902,6 +6903,7 @@ namespace Microsoft.FSharp.Core
         [<AutoOpen>]
         module ArrayExtensions =
             type ``[,,,]``<'T> with
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member arr.GetReverseIndex(dim: int, offset: int) = 
                     let len = 
                         match dim with
@@ -6914,6 +6916,7 @@ namespace Microsoft.FSharp.Core
                     len - offset - 1
 
             type ``[,,]``<'T> with
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member arr.GetReverseIndex(dim: int, offset: int) = 
                     let len = 
                         match dim with
@@ -6925,6 +6928,7 @@ namespace Microsoft.FSharp.Core
                     len - offset - 1
 
             type ``[,]``<'T> with
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member arr.GetReverseIndex(dim: int, offset: int) = 
                     let len = 
                         match dim with
@@ -6935,9 +6939,11 @@ namespace Microsoft.FSharp.Core
                     len - offset - 1
 
             type ``[]``<'T> with
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member arr.GetReverseIndex (_: int, offset: int) = arr.Length - offset - 1
 
             type String with
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member str.GetReverseIndex (_: int, offset: int) = str.Length - offset - 1
 
 namespace Microsoft.FSharp.Control

src/FSharp.Core/prim-types.fsi

@@ -2555,6 +2555,7 @@ namespace Microsoft.FSharp.Collections
         /// <param name="offset">The offset from the end.</param>
         ///
         /// <returns>The corresponding index from the start.</returns>
+        [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
         member GetReverseIndex: rank: int * offset: int -> int
 
         /// <summary>Returns a list with <c>head</c> as its first element and <c>tail</c> as its subsequent elements</summary>
@@ -4608,6 +4609,7 @@ namespace Microsoft.FSharp.Core
                 /// <param name="offset">The offset from the end.</param>
                 ///
                 /// <returns>The corresponding index from the start.</returns>
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member GetReverseIndex: rank: int * offset: int -> int
 
             type ``[,,]``<'T> with
@@ -4617,6 +4619,7 @@ namespace Microsoft.FSharp.Core
                 /// <param name="offset">The offset from the end.</param>
                 ///
                 /// <returns>The corresponding index from the start.</returns>
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member GetReverseIndex: rank: int * offset: int -> int
 
             type ``[,]``<'T> with
@@ -4626,6 +4629,7 @@ namespace Microsoft.FSharp.Core
                 /// <param name="offset">The offset from the end.</param>
                 ///
                 /// <returns>The corresponding index from the start.</returns>
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member GetReverseIndex: rank: int * offset: int -> int
 
             type ``[]``<'T> with
@@ -4635,6 +4639,7 @@ namespace Microsoft.FSharp.Core
                 /// <param name="offset">The offset from the end.</param>
                 ///
                 /// <returns>The corresponding index from the start.</returns>
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member GetReverseIndex: rank: int * offset: int -> int
 
             type System.String with
@@ -4644,6 +4649,7 @@ namespace Microsoft.FSharp.Core
                 /// <param name="offset">The offset from the end.</param>
                 ///
                 /// <returns>The corresponding index from the start.</returns>
+                [<Experimental("Experimental library feature, requires '--langversion:preview'")>]
                 member GetReverseIndex: rank: int * offset: int -> int
 
         /// <summary>A module of compiler intrinsic functions for efficient implementations of F# integer ranges