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Don't re-infer compiled CodeInstance from external images (#61479)

998cb27 introduced a regression that, when invoking a CodeInstance
from an external image, would result in the generation of a tojlinvoke
trampoline for every invoke but the first (in the order `compile!`
infers them).

```julia
module P2

function what(x)
    try
        show(x)
    catch e
        rethrow()
    end
end

what(1) # calls julia_rethrow_88_gfthunk
what(:a) # calls julia_rethrow_88_gfthunk
what('a') # calls jlpkg_rethrow_0

end # module P2
```

After inferring the first use of `rethrow` in this example, `compile!` would
find no source for the CodeInstance and infer it again, with
`source_mode=SOURCE_MODE_GET_SOURCE`.  It then inserts the resulting new
CodeInstance into the `code_cache`.  Before 998cb27, this went into the
InternalCodeCache, which would insert it after any CodeInstances with the
`invoke` field set.  Now, it is inserted into the OverlayCodeCache, which takes
priority over the InternalCodeCache.  For the two remaining specializations of
`what`, this new CodeInstance is used for the invoke.

Since it's a waste to re-infer this CodeInstance when we have
`external_linkage`, it makes more sense to thread this through to `compile!`
and skip any CodeInstances that are from the image.

Thanks to mlechu for helping track down this issue and providing the
MWE.

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