Document breaking change in #5712 (#11399)

Adds a file to the C# compiler docs that lists known breaking changes with the native compiler and
adds #5712 to that list.

docs/compilers/CSharp/Compiler Breaking Changes.md

+**This document lists known breaking changes in Roslyn (VS2015+) from the native C# compiler (VS2013 and previous).**
+
+*Breaks are formatted with a monotonically increasing numbered list to allow them to referenced via shorthand (i.e., "known break #1").
+Each entry should include a short description of the break, followed by either a link to the issue describing the full details of the break or the full details of the break inline.*
+
+1. In some cases, due to a bug in the native compiler, programs with pointers to structs with one or more
+   type parameters compiled without error. All such programs should now produce errors in Roslyn. See
+   [#5712](https://github.com/dotnet/roslyn/issues/5712) for examples and details.

src/Compilers/CSharp/Test/Semantic/Semantics/UnsafeTests.cs

@@ -7681,6 +7681,27 @@ unsafe static void Main()
 
         #region PointerTypes tests
 
+        [WorkItem(5712, "https://github.com/dotnet/roslyn/issues/5712")]
+        [Fact]
+        public void PathalogicalRefStructPtrMultiDimensionalArray()
+        {
+            var text = @"
+class C
+{
+  class Foo3 { 
+     internal struct Struct1<U> {} 
+  }
+
+  unsafe void NMethodCecilNameHelper_Parameter_AllTogether<U>(ref Foo3.Struct1<int>**[][,,] ppi) { }
+}
+";
+            CreateCompilationWithMscorlib(text, options: TestOptions.UnsafeDebugDll).VerifyDiagnostics(
+                // (8,67): error CS0208: Cannot take the address of, get the size of, or declare a pointer to a managed type ('C.Foo3.Struct1<int>')
+                //   unsafe void NMethodCecilNameHelper_Parameter_AllTogether<U>(ref Foo3.Struct1<int>**[][,,] ppi) { }
+                Diagnostic(ErrorCode.ERR_ManagedAddr, "Foo3.Struct1<int>*").WithArguments("C.Foo3.Struct1<int>").WithLocation(8, 67));
+        }
+
+
         [WorkItem(543990, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/543990")]
         [Fact]
         public void PointerTypeInVolatileField()