Do not populate SourceEntries in AddSourceFile.

It only generates a default constructed SourceEntry for each
cmSourceFile.

  this->Internal->SourceEntries[sf];

in cmTargetTraceDependencies::Trace has the same effect, and is called
early in generation-time for each source file anyway.
Move TraceDependencies to cmGeneratorTarget.
Merge topic 'use-generator-target' into next

07f5788 Move TraceDependencies to cmGeneratorTarget.
fa03777 Do not populate SourceEntries in AddSourceFile.