go/types: report correct argument types for make() built-in calls

Change Checker.index to return the type and constant index value
rather than just a boolean valid flag and the constant value.
While at it, rename some variables and simplify the control flow.

Adjust all uses of Checker.index to new signature. In code for
make() built-in, collect type information for signature reporting.

Fixes #37393.

Change-Id: Id70196faa9539ed5a0d6b59e0f3ea05e05f2f6a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/220585


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>