[dev.typeparams] cmd/compile/internal/importer: review of support.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/support.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: Icb8e7e7cac02751265c1020431018293826bad18
Reviewed-on: https://go-review.googlesource.com/c/go/+/324130


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