fix the error span when accessing tuple/struct fields in a constant

[oli-obk]

previously the error was erased and a non-const path error was emitted at the location of the field access instead of at the overflow location (as can be seen in the playground: http://is.gd/EuAF5F )

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[nikomatsakis]

@bors r+

[bors]

📌 Commit 8867593 has been approved by nikomatsakis

[nikomatsakis]

@bors r+

[bors]

📌 Commit 8867593 has been approved by nikomatsakis

[nikomatsakis]

(sorry for closing, clicked wrong button)

[bors]

⌛ Testing commit 8867593 with merge 5907957...

[bors]

⛄ The build was interrupted to prioritize another pull request.

[bors]

⌛ Testing commit 8867593 with merge ff140df...

[bors]

⛄ The build was interrupted to prioritize another pull request.

[bors]

⌛ Testing commit 8867593 with merge de62f9d...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt