Add hasCopyConstructor and hasPostblit traits

[adamdruppe]

I forked @edi33416 's PR here #10265 and split it into two pieces as mentioned in the comments.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20318	normal	Illegal instruction (core dumped)

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#10528"

[ghost]

The two traits can be written together with a small change

[ghost]

The two traits share most of their code. So here

if (e.ident == Id.hasPostblit || e.ident == Id.hasCopyConstructor)

[ghost]

and then here

 return (e.ident == Id.hasPostblit) ? (sd.postblit ? True() : False())
                                    : (sd.hasCopyCtor ? True() : False());

[adamdruppe]

yeah i originally literally just mindlessly copy/pasted it but this is nicer, change made just waiting on the autotester to make sure i didnt mess up

[wilzbach]

Please don't forget the dlang.org update.