Genex: Memoize usage requirement TARGET_PROPERTY existence

For each usage requirement (such as `INTERFACE_COMPILE_DEFINITIONS` or
`INTERFACE_INCLUDE_DIRECTORIES`), the value of the generator expression
`$<TARGET_PROPERTY:target,prop>` includes the values of the same
property from the transitive closure of link libraries of the target.

In cases that a target's transitive closure of dependencies does not
depend on the target being linked (the "head" target), we can memoize
whether or not a usage requirement property exists at all for that
target.  When a usage requirement does not exist for a target, we
can skip evaluating it for every consuming target.

Fixes: #18964, #18965

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