debuginfo: emit DW_TAG_call_site entries

[resrever]

Set FlagAllCallsDescribed on function definition DIEs so LLVM emits DW_TAG_call_site entries, letting debuggers and analysis tools track tail calls.

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.