Fix nullness flow for objnull from failwith after null pattern

[Copilot]

Problem

When nullness checking is enabled, variables in non-null branches of match expressions should be refined to non-null types, but type aliases were not being preserved during this refinement process.

Consider this example:

type objnull = obj | null

let getEnvAliasObj (_: string) : objnull = failwith "stub"

let valueAliasObj =
    match getEnvAliasObj "ENVVAR" with
    | null -> "missing"
    | x -> x.ToString() // x should be refined to obj (non-null), preserving the alias

Previously, the type alias objnull would be stripped during nullness refinement, and the variable x would lose its connection to the original type alias.

Root Cause

The issue was in the removeNull function within TcMatchClause in CheckExpressions.fs. The original implementation used:

let removeNull t =
    let stripped = stripTyEqns cenv.g t
    replaceNullnessOfTy KnownWithoutNull stripped

This approach stripped type abbreviations using stripTyEqns and then applied the nullness refinement to the stripped type, losing the original type alias information.

Solution

Modified the nullness refinement logic to:

1. Use stripTyEqns only for decision-making (to determine if refinement is applicable)
2. Apply the refinement directly to the original type structure to preserve aliases
3. Manually construct refined types with KnownWithoutNull nullness while preserving the original type constructor and type arguments

The new implementation:

let removeNull t =
    match stripTyEqns cenv.g t with
    | TType_app (tcref, _, _) when not tcref.Deref.IsStructOrEnumTycon ->
        match t with
        | TType_app (tcrefOrig, tinstOrig, _) -> TType_app (tcrefOrig, tinstOrig, KnownWithoutNull)
        | _ -> replaceNullnessOfTy KnownWithoutNull t
    | TType_var _ ->
        match t with  
        | TType_var (tpOrig, _) -> TType_var (tpOrig, KnownWithoutNull)
        | _ -> replaceNullnessOfTy KnownWithoutNull t
    | _ -> t

Impact

• ✅ Type aliases like objnull = obj | null are now preserved in non-null branches
• ✅ Variables in non-null match branches are correctly refined to non-null types
• ✅ Handles both defaulting cases (failwith "") and explicit type aliases
• ✅ Maintains compatibility with existing nullness checking functionality

Testing

Added comprehensive test cases covering:

• Defaulting case where failwith "" creates unconstrained types
• Explicit type aliases to obj | null
• Explicit type aliases to string | null

Fixes #18488

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