Fix handling generic custom attributes

[MichalStrehovsky]

Fixes#78200.

Cc @dotnet/ilc-contrib

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Issue Details

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Fixes #78200.

Author:	MichalStrehovsky
Assignees:	-
Labels:	area-NativeAOT-coreclr
Milestone:	-

[jkotas]

Failure is fixed by #78296

[MichalStrehovsky]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/3458321722

[agocke]

Are we expecting the linked PR to unblock the unit test? Should we port that change back to 7.0 as well?

[MichalStrehovsky]

Are we expecting the linked PR to unblock the unit test? Should we port that change back to 7.0 as well?

• The linked pull request ([mono] Fix support for generic custom attributes. #78091) originally only had product change and no test changes.
• I suggested a regression test because that's what we usually do for customer-reported issues that don't have a failing test ([mono] Fix support for generic custom attributes. #78091 (comment)).
• Such test would contribute to Generic attributes lack testing #73735 because we lack any testing for generic attributes in the libraries tests (and we almost shipped illinker with broken generic attributes support too) - we only found out when generic attribute tests were being added to S.R.Metadata.
• The test was added and found the NativeAOT bug [mono] Fix support for generic custom attributes. #78091 (comment)
• It was then pointed out that CoreCLR partition has some generic attribute tests written in IL that Mono disabled long time ago.
• The newly added test was deleted (sigh) and the CoreCLR test was unblocked instead.
• We don't run CoreCLR tests with NativeAOT yet.
• We also don't run CoreCLR tests with IL linker.
• Generic attributes lack testing #73735 still tracks adding proper libraries testing (the CoreCLR test should ideally be deleted. We do thorough API testing in the libraries partition).

TL;DR: the linked pull request could have contributed a useful test for this, but doesn't anymore.