Unnecessary concatenation of string literals

[felipebz]

It's not necessary concatenate two string literals at runtime.

Noncompliant code:

var := 'a'||'b';
var2 := concat('a', 'b');

Compliant code:

var := 'ab';
var2 := 'ab';

[felipebz]

Exception: it shouldn't register an issue if the two string are on different lines:

var := 'aaaa'||
       'bbb';