[Automated] PRs inserted in VS build main-12211.18

[View Complete Diff of Changes](https://github.com/dotnet/fsharp/compare/ccd0aca57b751d13995ddd1396bfc84b677a40b9...4bb6faf078b467ee53ab6006e47b928a94c65cf1?w=1)
- [Remove always-on language feature flag NonVariablePatternsToRightOfAsPatterns (#20158)](https://github.com/dotnet/fsharp/pull/20440)
- [Remove always-on DotlessFloat32Literal language feature flag](https://github.com/dotnet/fsharp/pull/20441)
- [[MicroPerf] Reverse instead of sort already-ordered branch fixups in IL writer](https://github.com/dotnet/fsharp/pull/20349)
- [Fail F# tests whose return value xUnit never executes](https://github.com/dotnet/fsharp/pull/20339)
- [[MicroPerf] Eliminate closure allocations in List.mapq and List.lengthsEqAndForall2](https://github.com/dotnet/fsharp/pull/20372)
- [Improve parser error recovery for underscore-dot patterns](https://github.com/dotnet/fsharp/pull/20453)
- [Remove always-on ExtendedStringInterpolation language feature flag](https://github.com/dotnet/fsharp/pull/20431)
- [Remove always-on InterfacesWithAbstractStaticMembers feature flag](https://github.com/dotnet/fsharp/pull/20432)
- [Make List.vMapFold inline to drop the nullness-import closure](https://github.com/dotnet/fsharp/pull/20415)
- [[MicroPerf] Eliminate closure allocations in well-known-attribute queries](https://github.com/dotnet/fsharp/pull/20423)
- [Remove per-call closure allocations from post-inference type checks](https://github.com/dotnet/fsharp/pull/20447)
- [[MicroPerf] Inline the free-variable typar foldBacks](https://github.com/dotnet/fsharp/pull/20385)
- [chore(Test): Rollback precautionary serial testing](https://github.com/dotnet/fsharp/pull/20343)
- [feat(Task, Async): parallelLimit, sequential, startAsyncImmediate](https://github.com/dotnet/fsharp/pull/20294)
- [Cut allocations in the VS project options caches and drop a boxing check](https://github.com/dotnet/fsharp/pull/20413)
- [Keep final do! expressions on the Bind path](https://github.com/dotnet/fsharp/pull/20449)
- [Remove trailing whitespace from src and vsintegration/src](https://github.com/dotnet/fsharp/pull/20393)
- [Remove the ArithmeticInLiterals language feature flag](https://github.com/dotnet/fsharp/pull/20430)
- [Remove always-on ExtendedFixedBindings language feature flag](https://github.com/dotnet/fsharp/pull/20429)
- [Remove always-on language feature flag OverloadsForCustomOperations](https://github.com/dotnet/fsharp/pull/20427)
- [Avoid per-call closure allocation in type-hierarchy traversal](https://github.com/dotnet/fsharp/pull/20384)
- [Cache IL method parameter attributes during overload resolution](https://github.com/dotnet/fsharp/pull/20353)
- [Check let bindings through active patterns like a match (#16856)](https://github.com/dotnet/fsharp/pull/20383)
- [Remove trailing whitespaces from tests](https://github.com/dotnet/fsharp/pull/20355)

Start with the linked complete diff and review the individual pull requests listed in the issue to understand the changes included in build main-12211.18. Done means the batch is accounted for through those pull requests; this issue does not define a separate implementation task.

Written by the indexing model from the issue text.