Genex: Fix evaluation context propagation for TARGET_OBJECTS.

Extract a new method to encapsulate the requirements of evaluating
dependent-expressions, namely, propagation of the
EvaluateForBuildsystem setting, which is missing from the
getLinkedTargetsContent implementation.

Commit v3.1.0-rc1~688^2 (Genex: Only evaluate TARGET_OBJECTS to determine
target sources., 2014-03-20) introduced an error case for use of
TARGET_OBJECTS outside of the context of generating the buildsystem,
as the path to object files may be dependent on buildsystem
variables (See bug #15226).

Commit v3.1.0-rc1~314^2 (Allow INTERFACE_SOURCES to specify
$<TARGET_OBJECTS> (#14970), 2014-07-09) made it possible to
propagate such content to dependent targets.

While that commit propagated the EvaluateForBuildsystem setting
for the case of a TARGET_PROPERTY expression, as generated for
direct dependencies of a target in
cmTargetInternals::AddInterfaceEntries, it did not add propagation
for content from further transitive target dependencies, as determined
by getLinkedTargetsContent.
Merge topic 'fix-transitive-OBJECT_SOURCES-context' into next

672f1001 Genex: Fix evaluation context propagation for TARGET_OBJECTS.