cmGeneratorTarget: Fix link interface caching of partial results

`GetLinkInterface` and `GetLinkInterfaceLibraries` cache their results
to avoid repeating work.  In the case that the result does not depend on
the "head" target, they re-use results computed from the first call with
any "head" target.  However, if `GetLinkInterfaceLibraries` is called
first, then not all of the link interface is populated.  If
`GetLinkInterface` is later called, it needs to finish populating the
link interface even if a partially completed interface was cached.