Explain reason behind error span

We can't expand the span of the error reliably according to @oli-obk, so
just mention why it points to this particular expression.

src/librustc/mir/interpret/error.rs

@@ -399,7 +399,9 @@ pub fn description(&self) -> &str {
             RemainderByZero => "attempt to calculate the remainder with a divisor of zero",
             GeneratorResumedAfterReturn => "generator resumed after completion",
             GeneratorResumedAfterPanic => "generator resumed after panicking",
-            InfiniteLoop => "program will never terminate",
+            InfiniteLoop =>
+                "duplicate interpreter state observed while executing this expression, \
+                const evaluation will never terminate",
         }
     }
 }

src/test/ui/const-eval/infinite_loop.stderr

@@ -29,7 +29,7 @@ LL | |         //~| ERROR could not evaluate repeat length
 LL | |         let mut n = 113383; // #20 in A006884
 LL | |         while n != 0 { //~ ERROR constant contains unimplemented expression type
 LL | |             n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
-   | |                    ---------- program will never terminate
+   | |                    ---------- duplicate interpreter state observed while executing this expression, const evaluation will never terminate
 LL | |         }
 LL | |         n
 LL | |     }];