go/internal/gcimporter: add support for unified IR
This CL ports unified IR's types2 importer back to the go/types
API. Notably, it drops support for lazy importing, because those APIs
aren't exposed yet via go/types.
Also, it supports unified IR's "final" data format, which wholey
replaces the iexport data format rather than the current
backwards-compatible hack that cmd/compile uses. The next CL will
switch the compiler to using this same format.
Change-Id: I44e1744bbdc384c9c354119975e68befdc117cff
Reviewed-on: https://go-review.googlesource.com/c/go/+/386002
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>