[REG2.066a] Issue 12467 - char[] is implicitly convertible to string

https://issues.dlang.org/show_bug.cgi?id=12467

Refactoring part:
Unique expression conversion is always considered by `Expression::implicitConvTo`.
Therefore, testing it on their sub-expressions is enough in `Add/Mull/SliceExp`.

Regression fix:
Getting slice of static array is a conversion from value to reference. So if the sliced static array is an lvalue (means that it has external reference), unique conversion should be prevented.