Genex: Evaluate byproduct generator expressions in cmCustomCommandGenerator

Evaluate and expand generator expressions in byproducts of custom commands.

Note that it is still not possible to use generator expressions in byproducts
of the commands `add_custom_command` and `add_custom_target`.  These commands
still reject the input.  This is a preparation step for OUTPUT generator
expression support.

Issue: #12877
1 job for prepare-genex-in-byproducts
Status Job ID Name Coverage
  External
passed ghostflow-check-master