Flush PL_stashcache on glob-to-glob assignment

Flush PL_stashcache on glob-to-glob assignment

Commit dc93d7fb33f6b2093 says this:

    Flush PL_stashcache when assigning a file handle to a typeglob.

    File handles take priority over stashes for method dispatch.
    Assigning a file handle to a typeglob potentially creates a file
    handle where one did not exist before.  As PL_stashcache only con-
    tains entries for names which unambiguously resolve to stashes,
    such a change may mean that PL_stashcache now contains an invalid
    entry.  As it’s hard to work out exactly which entries might be
    affected, simply flush the entire cache and let it rebuild itself.

But it only applied to io-to-glob assignment.  This commit extends it
to glob-to-glob assignment.