Restore guard in ProcessMethodBody (#22990)

src/Compilers/Core/Portable/PEWriter/ReferenceIndexer.cs

@@ -119,6 +119,10 @@ protected override void ProcessMethodBody(IMethodDefinition method)
                         }
                     }
                 }
+                else if (!metadataWriter.MetadataOnly)
+                {
+                    throw ExceptionUtilities.Unreachable;
+                }
             }
         }