cmd/compile: extrapolate $GOROOT in unified IR

This ensures that diagnostics for files within $GOROOT continue to be
reported using their full filepath, rather than the abbreviated
filepath. Notably, this is necessary for test/run.go, which has tests
that expect to see the full filepath.

Updates #48247.

Change-Id: I440e2c6dd6109ca059d81cee49e476bba805d703
Reviewed-on: https://go-review.googlesource.com/c/go/+/348670


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>