transformer: fix a bug with string literal length (#14757)

vlib/v/gen/c/testdata/strlit_len_optimization.out

@@ -1,2 +1,4 @@
 5
 7
+6
+6

vlib/v/gen/c/testdata/strlit_len_optimization.vv

@@ -1,6 +1,10 @@
 fn main() {
 	a := 'Vlang'.len
 	b := r'Vlang\n'.len
+	c := 'Vlang\n'.len
+	d := 'Vlang$a'.len
 	println(a)
 	println(b)
+	println(c)
+	println(d)
 }

vlib/v/transformer/transformer.v

@@ -637,9 +637,11 @@ pub fn (mut t Transformer) expr(mut node ast.Expr) ast.Expr {
 		ast.SelectorExpr {
 			node.expr = t.expr(mut node.expr)
 			if mut node.expr is ast.StringLiteral && node.field_name == 'len' {
-				return ast.IntegerLiteral{
-					val: node.expr.val.len.str()
-					pos: node.pos
+				if !node.expr.val.contains('\\') || node.expr.is_raw {
+					return ast.IntegerLiteral{
+						val: node.expr.val.len.str()
+						pos: node.pos
+					}
 				}
 			}
 		}