Generator expressions in a non-cross custom command's `COMMAND`
arguments are evaluated in the command config. Target-level
dependencies implied by `TARGET_FILE` must therefore be cross
dependencies. This is important to generate proper target-level
dependencies on the cross-config build statements for the target to
which the custom command is attached.
Fixes: #22855
With generator expressions in a custom command's `OUTPUT` and
`BYPRODUCTS`, it is possible to have no outputs at all for a particular
configuration. Generate no rule in this case.
Fixes: #21989
