checker: prevent C error on const mutation

vlib/v/checker/checker.v

@@ -502,6 +502,8 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) {
 					c.error('`$it.name` is immutable, declare it with `mut` to make it mutable',
 						it.pos)
 				}
+			} else if it.name in c.const_names {
+				c.error('cannot assign to constant `$it.name`', it.pos)
 			}
 		}
 		ast.IndexExpr {

vlib/v/checker/tests/const_field_add_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/const_field_add_err.v:6:2: error: cannot assign to constant `a` 
+    4 | 
+    5 | fn main() {
+    6 |     a += 1
+      |     ^
+    7 | }

vlib/v/checker/tests/const_field_add_err.vv

@@ -0,0 +1,7 @@
+const (
+	a = 1
+)
+
+fn main() {
+	a += 1
+}

vlib/v/checker/tests/const_field_dec_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/const_field_dec_err.v:6:2: error: cannot assign to constant `a` 
+    4 | 
+    5 | fn main() {
+    6 |     a--
+      |     ^
+    7 | }

vlib/v/checker/tests/const_field_dec_err.vv

@@ -0,0 +1,7 @@
+const (
+	a = 1
+)
+
+fn main() {
+	a--
+}

vlib/v/checker/tests/const_field_inc_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/const_field_inc_err.v:6:2: error: cannot assign to constant `a` 
+    4 | 
+    5 | fn main() {
+    6 |     a++
+      |     ^
+    7 | }

vlib/v/checker/tests/const_field_inc_err.vv

@@ -0,0 +1,7 @@
+const (
+	a = 1
+)
+
+fn main() {
+	a++
+}

vlib/v/checker/tests/const_field_sub_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/const_field_sub_err.v:6:2: error: cannot assign to constant `a` 
+    4 | 
+    5 | fn main() {
+    6 |     a -= 1
+      |     ^
+    7 | }

vlib/v/checker/tests/const_field_sub_err.vv

@@ -0,0 +1,7 @@
+const (
+	a = 1
+)
+
+fn main() {
+	a -= 1
+}