cmd/vet: recognise func type conversions

cmd/vet: recognise func type conversions

In hasSideEffects, vet has to be taught whether or not a CallExpr is an
actual function call, or just a type conversion.

The previous code knew how to differentiate fn(arg) from int(arg), but
it incorrectly saw (func(T))(fn) as a func call. This edge case is
slightly tricky, since the CallExpr.Fun has a func signature type, just
like in func calls.

However, the difference is that in this case the Fun is a type, not a
value. This information is in types.TypeAndValue, so use it.

Change-Id: I18bb8b23abbe7decc558b726ff2dc31fae2f13d6
Reviewed-on: https://go-review.googlesource.com/111416
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>