misc/cgo: Add test for C union read/write

[howjmay]

Add test for reading and writing C language union datatype

Fixes #39537

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Message from Ian Lance Taylor:

Patch Set 1:

(3 comments)

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This PR (HEAD: a4e9891) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/248683 to see it.

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This PR (HEAD: 5da8cb0) has been imported to Gerrit for code review.

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Message from Hau Yang:

Patch Set 3:

(3 comments)

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After addressing review feedback, remember to publish your drafts!

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This PR (HEAD: aa0bb07) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/248683 to see it.

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Message from Hau Yang:

Patch Set 4:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 5:

(2 comments)

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Message from Hau Yang:

Patch Set 4:

(1 comment)

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After addressing review feedback, remember to publish your drafts!

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Message from Ian Lance Taylor:

Patch Set 5:

(2 comments)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/248683.
After addressing review feedback, remember to publish your drafts!