cmd/compile: extrapolate $GOROOT in unified IR
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 <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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