Translate user defined moves

[lucic71]

This PR translates move constructors as ordinary constructors: fn S_pmutS(_a0: *mut S) -> Self.

The rules for translating and using the move constructors are:

No	Move Constructor (MC)	Copy Constructor	Result
1	user-written	implicit/defaulted/deleted	translate MC and always use it
2	user-written	user-written	translate MC and always use it
3	implicit/defaulted	implicit/defaulted	don't translate MC and use .clone() on move
4	implicit/defautled	user-written/deleted	translate MC and always use it

For 1 and 2, we always use the translated move constructor because it might contain custom logic that needs to run on every move. For 3 we use .clone() because it has the same effect as a move would have. For 4 .clone() is not available (deleted) or might contain custom logic (user-written) so we synthesize an implicit/defaulted move constructor and use it.

std::move becomes a transparent construct, i.e. no move decision is taken at the time of the cast. It's only used by clang to choose the right constructor. Moves happen on CXXConstructExpr.

Each STL type has move construction rules. Rules that move using std::mem::take have their bodies rewritten as follows:

• record has move constructor => rewrite std::mem::take as a call to the move constructor
• record does not have move constructor and is copyable => convert as rvalue (without clone)
• record does not have move constructor and is not copyable => convert as fresh rvalue (with clone)
• record is STL type => use the rule for STL move constructor
• temporary value => convert as rvalue

[nunoplopes]

the translation of this is calling the copy constructor rather than move, I think.

[lucic71]

It's the move constructor. Copy constructor is deleted because Holder contains unique_ptr which is not copy

[nunoplopes]

This strategy works while fields are boxed in Values, but otherwise we need move constructors to call std::mem::take to move objects and destroy (default initialize) the old one, otherwise we break the translation of classes for example that do reference counting.
I would also like to see more meaningful names in functions. It's very hard to follow the generated code.