cmGeneratorExpressionNode: implement `COMPILE_ONLY` genex

This generator expression is the inverse of `LINK_ONLY` and only coveys
usage requirements for the purposes of compilation. Its intended use is
to avoid needing to export targets that do not have link usage
requirements (e.g., header-only libraries) when used by another target.

It will also be used to represent private usage requirements on exported
C++ module-containing targets in the future.

Eventually there should be logic to collapse nesting of
`$<COMPILE_ONLY>` and `$<LINK_ONLY>` when generating instances of
either. A TODO is left in the code for this case.

See: #15415

0 files changed, 0 insertions, 0 deletions