go/ssa: use valid types for synthetic values

[cpunion]

Range iterator and defer stack values currently use a private
opaqueType. It is not a valid go/types type, so ordinary type utilities
may panic and clients must detect it through implementation details.
LLGo currently uses reflection for this purpose.

Use inaccurate but valid Go types instead: () for range iterators and
*struct{} for defer stacks. These are the directions suggested in the
existing issue discussions. Remove the now-unneeded substitution and
sanity-check exceptions, and verify both values work with typeutil.Map.

This would let LLGo remove its private-type reflection workaround added
in xgo-dev/llgo#1826.

Fixes golang/go#19670.
Fixes golang/go#72914.

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[gopherbot]

Message from Alex Putman:

Patch Set 1: Commit-Queue+1

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-08-21T17:06:07Z","revision":"1d80138628016bd19327318b479025125c7fe02a"}

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Message from Alex Putman:

Patch Set 1: -Commit-Queue

(Performed by <GERRIT_ACCOUNT_60063> on behalf of <GERRIT_ACCOUNT_4809514>)

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 1:

This CL has failed the run. Reason:

Tryjob golang/try/x_tools-gotip-windows-386 has failed with summary (view all results):

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• go on master (commit f85791f) / tools on master (change 814561)

To reproduce, try gomote repro 8672840301939121233.

Additional links for debugging:

• test golang.org/x/tools/gopls module output

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 1: LUCI-TryBot-Result-1

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