cmd/compile/internal/syntax: set up dummy name and type if func name is missing

We do the same elsewhere (e.g. in parser.name when a name is missing).
This ensures functions have a (dummy) name and a non-nil type.
Avoids a crash in the type-checker (verified manually).
A test was added here (rather than the type checker) because type-
checker tests are shared between types2 and go/types and error
recovery in this case is different.

Fixes #63835.

Change-Id: I1460fc88d23d80b8d8c181c774d6b0a56ca06317
Reviewed-on: https://go-review.googlesource.com/c/go/+/538059
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

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