Allow INTERFACE_SOURCES to specify $<TARGET_OBJECTS> (#14970)

Fix cmTarget::GetSourceFiles to set EvaluateForBuildsystem on the
$<TARGET_PROPERTY:...,INTERFACE_SOURCES> generator expression so that
the $<TARGET_OBJECTS> generator expression is allowed within an
INTERFACE_SOURCES value.

Extend the InterfaceLibrary test to cover this case.  Extend the
RunCMake.TargetObjects test to cover failure of $<TARGET_OBJECTS>
when used through $<TARGET_PROPERTY:...,INTERFACE_SOURCES> in a
non-buildsystem context.

1 files changed, 1 insertions, 0 deletions

diff --git a/Tests/InterfaceLibrary/obj.cpp b/Tests/InterfaceLibrary/obj.cpp
new file mode 100644
index 0000000..ee6f5fe
--- /dev/null
+++ b/Tests/InterfaceLibrary/obj.cpp
@@ -0,0 +1 @@
+int obj() { return 0; }