[Aghayenoor]

Decode line directive offsets in unified export data so that imported
objects point to their original cgo source lines. Keep high line
numbers in fixed-size synthetic file ranges that share a line table.

Add a go/packages regression test for cgo declarations at an ordinary
source line and at lines 70000 and 70001, in module and GOPATH mode.

Tests passed with Go 1.27.3 for go/packages, internal/gcimporter, and
go/gcexportdata.

Fixes golang/go#74037

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gopherbot]

This PR (HEAD: 66bee2c) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/tools/+/860501.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.