Make $<LINK_ONLY> available to projects (#14751)

Previously this generator expression was used internally by the
target_link_libraries command to honor private linking requirements of
static libraries in their INTERFACE_LINK_LIBRARIES.  Remove the check
that limits $<LINK_ONLY> to this use case to make it available for
project code to use too.
Honor $<LINK_ONLY> when checking interface properties

Callers of cmTarget::GetLinkImplementationClosure are interested in the
set of targets whose interface properties propagate to the current
target.  This excludes targets guarded by $<LINK_ONLY>.
Merge topic 'generalize-LINK_ONLY' into next

8722b540 Honor $<LINK_ONLY> when checking interface properties
1657c4a5 Make $<LINK_ONLY> available to projects (#14751)