Fix Bugzilla 14945 - unions are missing from the ABI page

[ntrel]

Also list empty struct size and add example.

[dlang-bot]

Thanks for your pull request and interest in making D better, @ntrel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	14945	trivial	unions are missing from the ABI page

[Bolpat]

Avoid the double negation “no … with non-…” and instead use “all … with …”.

Suggested change

	$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with no fields of non-zero
	$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with all fields of zero

Mathematically speaking, any all statement about things is vacuously true if there are none. Most people who read this part of the spec will understand it, but if you’re in doubt, you could do this instead:

Suggested change

	$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with no fields of non-zero
	$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with no fields or all fields of zero

[Bolpat]

Also extern(C++) structs seem to have non-zero size judging by running a simple example. The C++ Standard (all versions) agrees with that.

Suggested change

	* An `extern(D)` struct or union with no fields has size 1, not 0.
	* An `extern(D)` or `extern(C++)` struct or union with no fields has size 1, not 0.

[ntrel]

This paragraph is about differences from C. The link that follows is for further information.

[Bolpat]

Might be worth adding an extern(C++) example.

Suggested change

	extern(C) struct C {}
	struct D {}
	static assert(C.sizeof == 0);
	static assert(D.sizeof == 1);
	extern(C) struct C {}
	extern(C++) struct Cpp {}
	struct D {}
	static assert(C.sizeof == 0);
	static assert(Cpp.sizeof == 1);
	static assert(D.sizeof == 1);