Restore support for precompiled headers with unity builds
Re-implement !9118 (merged) using a simpler approach that also preserves support for PCH with Unity builds.
Fixes: #25650 (closed)
Backport: release
Re-implement !9118 (merged) using a simpler approach that also preserves support for PCH with Unity builds.
Fixes: #25650 (closed)
Backport: release