Merge topic 'restore-pch-with-unity' into release-3.28

30829da506 Unity: Clarify source comments on unity build transformation
87bf1c6c33 Merge branch 'unity-after-compile-features' into restore-pch-with-unity
df08c37a42 cmGlobalGenerator: Add unity/pch sources after computing compile features
004c3c3986 Tests: Add case covering PCH in a unity build

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Acked-by: Ben Boeckel <ben.boeckel@kitware.com>
Merge-request: !9215