SMTChecker: fixed struct constructor where FixedBytes member is initialized with a string literal

test/libsolidity/smtCheckerTests/types/struct/struct_constructor_fixed_bytes_from_string_1.sol

@@ -0,0 +1,14 @@
+pragma experimental SMTChecker;
+contract C {
+  struct S {
+    int a;
+    bytes5 b;
+  }
+  function f() public pure {
+    assert(S({a:2, b:""}).b == bytes5(0)); // should hold
+    assert(S({a:2, b:""}).a == 0); // should fail
+  }
+}
+// ----
+// Warning 5523: (0-31): The SMTChecker pragma has been deprecated and will be removed in the future. Please use the "model checker engine" compiler setting to activate the SMTChecker instead. If the pragma is enabled, all engines will be used.
+// Warning 6328: (178-207): CHC: Assertion violation happens here.\nCounterexample:\n\n\nTransaction trace:\nC.constructor()\nC.f()