cgen: fix sizeof('str') and sizeof(r'str') (#14507)

vlib/v/gen/c/cgen.v

@@ -5450,12 +5450,14 @@ fn (mut g Gen) size_of(node ast.SizeOf) {
 		g.error('unknown type `$sym.name`', node.pos)
 	}
 	if node.expr is ast.StringLiteral {
+		if node.expr.language == .c {
 			g.write('sizeof("$node.expr.val")')
-	} else {
+			return
+		}
+	}
 	styp := g.typ(node_typ)
 	g.write('sizeof(${util.no_dots(styp)})')
 }
-}
 
 fn (mut g Gen) enum_val(node ast.EnumVal) {
 	// g.write('${it.mod}${it.enum_name}_$it.val')

vlib/v/tests/sizeof_test.v

@@ -25,5 +25,6 @@ fn test_sizeof() {
 	assert sizeof(flag.Flag) > 4
 
 	assert sizeof(c'hello') == 6
-	assert sizeof(r'hello') == 6
+	assert sizeof(r'hello') == 16
+	assert sizeof('hello') == 16
 }