This test verifies that we can export constants with unknown kind.
Previously, the exporter would panic while attempting to convert such constants
to their target type (float64, in this case).

-- flags --
-min_go=go1.20

-- go.mod --
module mod.txt/p

go 1.20
-- p.go --
package p

const EPSILON float64 = 1e- //@diag(re"1e-()", re"exponent has no digits")