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

Source/cmTargetLinkLibrariesCommand.cxx

@@ -552,6 +552,7 @@ bool TLL::HandleLibrary(ProcessingState currentProcessingState,
       currentProcessingState == ProcessingPlainPrivateInterface) {
     if (this->Target->GetType() == cmStateEnums::STATIC_LIBRARY ||
         this->Target->GetType() == cmStateEnums::OBJECT_LIBRARY) {
+      // TODO: Detect and no-op `$<COMPILE_ONLY>` genexes here.
       std::string configLib =
         this->Target->GetDebugGeneratorExpressions(lib, llt);
       if (cmGeneratorExpression::IsValidTargetName(lib) ||