cmd/compile/internal/types2: simplify Checker.Call

Now that we use square brackets for instantiations, we
can tell type arguments from ordinary arguments without
"guessing" which permits a simpler implementation.

Specifically, replace use of Checker.exprOrTypeList with
Checker.exprList, and delete Checker.exprOrTypeList and
Checker.multiExprOrType.

Disable a test for an (esoteric) failure due to an
unrelated problem with error matching when running
the test.

Change-Id: I17f18fffc32f03fa90d93a68ebf56e5f2fcc9dab
Reviewed-on: https://go-review.googlesource.com/c/go/+/306171


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