Unity Build: Fix per-config sources in multi-config generators
Single-config generators already support unity builds with per-config
sources because they compute sources using CMAKE_BUILD_TYPE
as the
configuration. Each original source is either included in the unity
build source, or not.
Teach multi-config generators to compute the list of sources for inclusion in unity builds using all configurations. Previously they only used the empty string as the configuration. Each original source may be included in some configurations, but not others. Use preprocessor conditions to guard their inclusion when necessary.
Fixes: #22892 (closed)