[dev.typeparams] cmd/compile: extract SetBaseTypeIndex function

The unified IR importer requires a way to set symbol indices for
imported types, so provide an exported API for this.

Change-Id: I2f088628f56d5b9f1097196dc1aa23f0a8b8d496
Reviewed-on: https://go-review.googlesource.com/c/go/+/327050


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>