Do not export INTERFACE_LINK_LIBRARIES from non-linkable targets

When cmExportFileGenerator::SetImportLinkInterface exports the old
LINK_INTERFACE_LIBRARIES property values it skips doing so for
non-linkable targets because target->GetLinkInterface returns NULL for
such targets.

Since cmExportFileGenerator::PopulateInterfaceLinkLibrariesProperty
looks at the INTERFACE_LINK_LIBRARIES property directly instead of using
the computed link interface, teach it to skip exporting the property if
target->IsLinkable returns false.

Extend the RunCMake.CMP0022 test with a case covering this.  Simply
export an executable target that links to a library that is not
exported.
Merge branch 'policy-CMP0022-fixes' into policy-CMP0022-fixes-for-master
Merge topic 'policy-CMP0022-fixes-for-master' into next

e9f353a Merge branch 'policy-CMP0022-fixes' into policy-CMP0022-fixes-for-master
23d21b7 Do not export INTERFACE_LINK_LIBRARIES from non-linkable targets