[expr.const] Use different classes for unrelated parts of the example

[jakubjelinek]

Fixes #8425.

[tkoeppe]

Can you please update both the commit description (as per https://github.com/cplusplus/draft/wiki/Commit-message-format) and the first post (state Fixes #nnnn.)?

Note in particular that the commit message should not include any GitHub maintenance metadata.

[jensmaurer]

For future pull requests, please see the help for the commit message format here: https://github.com/cplusplus/draft/wiki/Commit-message-format
(in particular, see "editorial commits")

In this case, we want something like

[expr.const] Use different classes for unrelated parts of the example

The "fixes" or "resolves" for the github machinery only goes into the pull request description, not into the commit message.

Please fix and force-push; thanks for your help!