C#: Delete exception splitting.

[aschackmull]

I believe that exception splitting adds almost zero value, so this PR simply gets rid of it.

[aschackmull]

Dca looks good. dotnet__aspnetcore has one FP removed for cs/constant-condition, and all other result changes are confined to mono in buildmode=none. I've gone through a lot of these, there are a few precision increases, and a number of new FPs, but in all cases it appears that the changes arise from previous mishandling of splitting.

[Copilot]

Pull Request Overview

This pull request removes exception splitting functionality from the C# control flow graph analysis, treating all exceptions uniformly instead of differentiating by exception type.

Key Changes:

• Removed exception handler splitting logic that tracked specific exception types through catch clauses
• Updated test expectations to reflect simplified exception handling in control flow graphs
• Eliminated exception type annotations from SSA definitions and control flow nodes

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
csharp/ql/lib/semmle/code/csharp/controlflow/internal/Splitting.qll	Removed ExceptionHandlerSplitting module and TExceptionHandlerSplitKind type
csharp/ql/lib/semmle/code/csharp/controlflow/ControlFlowGraph.qll	Removed ExceptionHandlerSplit class export
csharp/ql/test/library-tests/exceptions/Exceptions1.expected	Added new exception flow edges that were previously filtered by exception type
csharp/ql/test/library-tests/dataflow/ssa/*.expected	Removed exception type annotations from SSA definitions
csharp/ql/test/library-tests/controlflow/graph/*.expected	Updated control flow graph test expectations to remove exception type splits
csharp/ql/test/library-tests/assignables/AssignableDefinitionNode.expected	Removed exception type annotation from assignable definition
csharp/ql/test/library-tests/csharp8/switchexprcontrolflow.expected	Simplified exception control flow in switch expressions

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[michaelnebel]

LGTM!
Thx!