[dev.typeparams] cmd/compile/internal/importer: review of gcimporter.go
[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 <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,golang/go