unstably allow constants to refer to statics and read from immutable statics

compiler/rustc_error_codes/src/error_codes/E0013.md

@@ -1,9 +1,11 @@
+#### Note: this error code is no longer emitted by the compiler
+
 Static and const variables can refer to other const variables. But a const
 variable cannot refer to a static variable.
 
 Erroneous code example:
 
-```compile_fail,E0013
+```compile_fail,E0658
 static X: i32 = 42;
 const Y: i32 = X;
 ```