[dev.typeparams] cmd/compile/internal/importer: review of gcimporter.go

This CL removes the // UNREVIEWED disclaimer at the top of the
file. This file is essentially a copy of its reviewed version
at src/go/internal/gcimporter/gcimporter.go with adjustments to
make it work for the compiler and types2. To see the changes
made with respect to the original, compare patchset 1 against
patchset 2.

Change-Id: I0fd635730fb6bdee8cef1b89154f4049a6581751
Reviewed-on: https://go-review.googlesource.com/c/go/+/324132


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