optimize memory layout of aggregates

[ghost]

On x86_64, StructDeclaration stood on 569 bytes, now 560, which saves allocation of a small block, simply by reordering fields.
Note that this time the gain is always negligible as you dont have thousands of different struct in a project.
My methodology to test (/usr/bin/time -v) doesnt even show anything tangible.

[dlang-bot]

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

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

dub run digger -- build "master + dmd#11370"

[thewilsonator]

Headers.

[ghost]

Headers ok now

[thewilsonator]

Looks good aside the merge conflict

[ghost]

conflicts solved

[ibuclaw]

This caused a regression.

$ gdc a.d
d21: error: struct ‘gcc.builtins.__va_list_tag’ circular or forward reference

[ghost]

bad headers ?

[ghost]

can be caused by the rebase too ?

[ibuclaw]

Yes, bad headers. It could have been avoided if tests are added whenever public fields get moved around.

#11394

[ibuclaw]

can be caused by the rebase too ?

Can't say. But they are clearly in the wrong order when comparing C++ and D definitions.

[ghost]

yeah anyway sorry, thanks for the fixup.