Since commit b80557c7 (file(GENERATE): Evaluate early to allow
generating source files, 2014-11-04, v3.2.0-rc1~398^2) we create the
`cmSourceFile` instances marked with a `GENERATED` source file property
before tracing source dependencies.  Move it to even earlier so that
steps in `cmGlobalGenerator::AddAutomaticSources` can operate on all
sources.  This also avoids the accidental `O(n^2)` calls for `n` local
generators that we had previously.

This is also needed since commit 83c1657f (Unity build: Generate
sources during Compute step, 2019-10-03) to support `file(GENERATE)`
outputs as sources in a target with `UNITY_BUILD` enabled.