Fix FS1182 not reported for unused let functions in class types

[T-Gro]

Summary

Fix #13849: FS1182 ("unused binding") warning was not reported for unused let function bindings inside class types.

Problem

Class-let function bindings are promoted to compiler-generated members, which sets IsCompiledAsTopLevel=true. The reportIfUnused helper in CheckIncrementalClasses.IncrClassReprInfo.ChooseRepresentation had a not v.IsCompiledAsTopLevel guard that silently suppressed FS1182 for these bindings.

Fix

Removed the not v.IsCompiledAsTopLevel guard from the local reportIfUnused helper. This helper is scoped to class-binding representation choice, so the change cannot introduce module-level false positives.

Test

Added test cases in tests/FSharp.Compiler.ComponentTests/CompilerOptions/fsc/warnon/warnon.fs verifying that FS1182 is correctly emitted for unused let functions in class types.

Fixes #13849

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Warning

No PR link found in some release notes, please consider adding it.

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md	No current pull request URL (#19805) found, please consider adding it

[T-Gro]

Review Summary

Looks good — Clean, well-scoped fix with thorough test coverage.

Analysis

The root cause is correctly identified: class let-functions are promoted to compiler-generated members via AdjustValToHaveValReprInfo (line 370), which sets ValReprInfo, making IsCompiledAsTopLevel = true. The old not v.IsCompiledAsTopLevel guard in reportIfUnused() then silently suppressed FS1182 for all class let-functions.

Why the fix is safe:

1. No module-level false positives — reportIfUnused() is local to IncrClassReprInfo.ChooseRepresentation, which only processes class-level bindings. Module-level bindings go through PostInferenceChecks.BindVal which has its own independent IsCompiledAsTopLevel guard (line 315 in PostInferenceChecks.fs).

2. No duplicate warnings — After AdjustValToHaveValReprInfo runs, the class let-function has IsCompiledAsTopLevel = true, so PostInferenceChecks.BindVal won't also emit FS1182 (its condition not v.IsCompiledAsTopLevel || topLevelBindingHiddenBySignatureFile() evaluates to false for class let-functions that aren't module bindings).

3. HasBeenReferenced is reliable — This flag is set during type-checking when the value is referenced, and applies uniformly to let-values and let-functions.

Tests

Excellent coverage:

• RED tests pin the bug fix (unused let-function variants, mixed used/unused, static let-function)
• GUARD tests prevent regressions (used functions, recursive self-reference, module-level, underscore-prefixed)

Minor

The release notes entry should include the PR link (as noted by the bot): [PR #19805](https://github.com/dotnet/fsharp/pull/19805).