Binary export format only.

Make sure we don't accidentally export an unnamed parameter
in signatures which expect all named parameters; otherwise
we crash during import. Appears to happen for _ (blank)
parameter names, as observed in method signatures such as
the one at: x/tools/godoc/analysis/analysis.go:76.

Fixes #15470.

TBR=mdempsky

Change-Id: I1b1184bf08c4c09d8a46946539c4b8c341acdb84
Reviewed-on: https://go-review.googlesource.com/22543Reviewed-by: Robert Griesemer <gri@golang.org>