checker: fix const type with raw string literal (#12761)

vlib/v/checker/checker.v

@@ -8732,7 +8732,7 @@ fn (mut c Checker) eval_comptime_const_expr(expr ast.Expr, nlevel int) ?ast.Comp
 			return expr.val.i64()
 		}
 		ast.StringLiteral {
-			return expr.val
+			return util.smart_quote(expr.val, expr.is_raw)
 		}
 		ast.CharLiteral {
 			runes := expr.val.runes()

vlib/v/tests/const_many_pluses_with_raw_string_literal_test.v

@@ -0,0 +1,29 @@
+const ca = r'x\n'
+
+const cb = 'x\n'
+
+const cc = ca + cb
+
+const cd = cc + cc
+
+const ce = cd + cd
+
+fn test_many_pluses_with_raw_string_literal() {
+	a := r'x\n'
+	assert a == ca
+	b := 'x\n'
+	assert b == cb
+	c := a + b
+	assert c == cc
+	d := c + c
+	assert d == cd
+	e := d + d
+	assert e == ce
+	println(e)
+	result := r'x\nx
+x\nx
+x\nx
+x\nx
+'
+	assert e == result
+}