cmd/compile/internal/types2: review of call.go

The changes between (equivalent, and reviewed) go/types/call.go
and call.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker, renaming of
sig_params to sigParams, and a couple of comment adjustments.
These additional changes reduce the difference between this
file and the go/types version.

Note that the verification pass using a MethodSet doesn't
exist because there's no MethodSet in types2.

Change-Id: I4d49460e0457401ed705dff5cfd17c9ff259d89f
Reviewed-on: https://go-review.googlesource.com/c/go/+/300998


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>