Fix infinite loop when accessing circular constants from inline assembly.

test/libsolidity/syntaxTests/inlineAssembly/circular_constant_access_module_err.sol

@@ -0,0 +1,17 @@
+==== Source: a ====
+import "b";
+uint constant c = d;
+==== Source: b ====
+import "a" as M;
+uint constant b = M.c;
+uint constant d = b;
+contract C {
+    uint constant a = b;
+    function f() public returns (uint t) {
+        assembly {
+            t := a
+        }
+    }
+}
+// ----
+// TypeError 3558: (b:178-179): Constant variable is circular.