Change all "unmanaged" (no GC fields) sequential types to have sequential layout.

[jkoritzinsky]

This is a prereq for #60639

This enables us to use the managed layout as the native layout for all LayoutKind.Sequential "C# unmanaged" structs, which is required for reconciling the "unmanaged" and "blittable" concepts.

Fixes #44579
Fixes #12977

[jkoritzinsky]

@jkotas I've updated this PR to now fix #12977. This is ready for another round of review.

[jkoritzinsky]

/azp run runtime-coreclr crossgen2 outerloop,runtime-coreclr crossgen2-composite

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[jkotas]

It looks good to me (once it builds on all platforms).

Are you happy with the test coverage we have for this, or do you plan to add more tests?

It would be useful to also get a review from @trylek or @davidwrighton .

[trylek]

I love this change, thanks so much Jeremy for finally putting some order to all this mess!

[jkoritzinsky]

/azp run runtime-coreclr crossgen2 outerloop,runtime-coreclr crossgen2-composite

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[jkoritzinsky]

/azp run runtime-coreclr crossgen2 outerloop,runtime-coreclr crossgen2-composite

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[jkoritzinsky]

It looks like the macOS ARM64 test failures are #62747 due to the following multidimensional array:

runtime/src/libraries/System.Console/src/System/ConsolePal.Unix.cs

Line 885 in fbabdb6

private static readonly string[,] s_fgbgAndColorStrings = new string[2, 16]; // 2 == fg vs bg, 16 == ConsoleColor values

[jkoritzinsky]

I've confirmed these failures are also in main now that #61938 has been merged long enough to have outerloop runs triggered against it.

I think the test coverage already have makes me comfortable enough to merge this PR. I'll add more test coverage when I implement #60639.