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.

Tests/InterfaceLinkLibraries/zot.cpp

@@ -0,0 +1,6 @@
+#include "zot.h"
+
+int zot()
+{
+  return 0;
+}