Allow delete on user-defined value types

[chriseth]

We do not allow any operator on user defined value types, but delete is special: It is implicitly used when we remove an array element and variables of user-defined value type have a default value - so

delete x; // x is of type Type

is essentially equivalent to

Type y;
x = y;

which means we already implicitly use it and it would be confusing to allow re-defining delete to something else.

[chriseth]

@cameel sees no reason not to allow it.

[cameel]

It's worth noting that it already works on UDVTs when they're a part of an array or a struct. It's only directly deleting them that is not allowed.

Also, this seems simple enough that I think we should consider making it a good first issue.

[casanova8172]

Fixed
#12273.txt
#12273
please check the code it is corrected or not?