Implement C++-like prvalue semantics for structs

[limepoutine]

This one is massive and messy (and hard to review, evidently). This PR ensures all construction in following forms emplaces directly to the storage, without creating any temporaries, matching C++17 semantics. Besides, this fixes a = b = S(1) being compiled as a blit instead of invoking copy constructor on a.

S s = S(1);
S s = b ? S(1) : S(2);
T t = T(S(1));
this.s = S(1);
return S(1);

What is still missing for a working C++17 std::basic_string binding:

• copy/move constructors
• remaining compiler-generated blits
• druntime hooks
• inliner

[dlang-bot]

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Testing this PR locally

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dub run digger -- build "master + dmd#22412"

[thewilsonator]

cc @WalterBright please review as this is alters the backend.

[WalterBright]

Is it possible to break this into multiple PRs? Such as one per each of the cases you mentioned in the opening comment?

[limepoutine]

OK, let me think it a bit...

[WalterBright]

thank you!