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.

Tests/InterfaceLibrary/CMakeLists.txt

@@ -18,8 +18,12 @@ set_property(TARGET imp::iface APPEND PROPERTY COMPATIBLE_INTERFACE_BOOL SOMEPRO
 set_property(TARGET imp::iface PROPERTY INTERFACE_SOMEPROP OFF)
 set_property(TARGET imp::iface PROPERTY INTERFACE_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/does_not_exist.cpp)
 
+add_library(objlib OBJECT obj.cpp)
+add_library(iface_objlib INTERFACE)
+target_sources(iface_objlib INTERFACE $<TARGET_OBJECTS:objlib>)
+
 add_executable(InterfaceLibrary definetestexe.cpp)
-target_link_libraries(InterfaceLibrary iface_nodepends headeriface subiface)
+target_link_libraries(InterfaceLibrary iface_nodepends headeriface subiface iface_objlib)
 
 add_subdirectory(libsdir)
 

Tests/InterfaceLibrary/definetestexe.cpp

@@ -15,9 +15,10 @@
 #error Expected IFACE_HEADER_BUILDDIR
 #endif
 
+extern int obj();
 extern int sub();
 
 int main(int,char**)
 {
-  return sub();
+  return obj() + sub();
 }

Tests/InterfaceLibrary/obj.cpp

@@ -0,0 +1 @@
+int obj() { return 0; }