Fix #7931: Resolve opened namespaces for attributes in recursive scopes

[Copilot]

Description

Fixes nested module attributes not being resolved in namespace rec / module rec scopes. Previously, using [<Extension>] on a module inside namespace rec would fail with FS0039: The type 'Extension' is not defined, even when open System.Runtime.CompilerServices was in scope.

Root Cause

In recursive scopes, open declarations are processed in Phase1AB — after Phase1A builds module/type entities. But module attributes are checked during Phase1A using envInitial, which doesn't yet include the opened namespaces. In non-recursive scopes, opens are processed sequentially before module attributes are checked, so this issue doesn't occur.

Changes Made

• src/Compiler/Checking/CheckDeclarations.fs: Added preProcessOpensForPhase1A helper that silently resolves open declarations before Phase1A attribute checking. Uses suppressErrorReporting and TemporarilySuspendReportingTypecheckResultsToSink to avoid duplicate diagnostics (since opens are re-processed properly in Phase1AB). Applied at two levels:
  1. Top-level opens in TcMutRecDefns_Phase1 (fixes namespace rec case)
  2. Inner opens in TcTyconDefnCore_Phase1A_BuildInitialModule (fixes nested module case)
• Regression tests: Added 8 test cases in AttributeResolutionInRecursiveScopes.fs covering the issue scenarios (Extension attribute on modules/types in namespace rec, nested modules, multiple opens, module rec, and non-recursive baseline).
• Release notes: Added entry to docs/release-notes/.FSharp.Compiler.Service/11.0.100.md.

Testing

• All 8 new regression tests pass
• All 158 existing custom attribute tests pass
• 636/638 broader conformance tests pass (2 failures are pre-existing and unrelated)

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Implemented the fix in commit d0e2dac (with release notes in c8d283c).

Root cause: In recursive scopes (namespace rec/module rec), open declarations were only processed in Phase1AB — after Phase1A builds module entities. But module attributes need opened namespaces during Phase1A.

Fix: Added preProcessOpensForPhase1A that silently resolves opens before Phase1A attribute checking, applied at both the top level and nested module level. All 158 custom attribute tests pass, plus 8 new regression tests for issue #7931.