C#: Prepare for pattern flow.

[michaelnebel]

In this PR we re-factor parts of the C# specific data flow and assignable definition.
It turns out that the existing logic contains some unwanted flow steps.

• There was flow from the matched expression in is expressions and switch/case statements to any sub-tuple pattern. However, there should only be a flow step to the immediate top-level pattern in match - flow to sub patterns should be handled as read steps.
• Local variable assignable definitions also contained sub-pattern variable declarations. Instead the PatternDefinition Assignable definition has been modified to include all sub pattern variable declarations. This will be needed in a subsequent PR.

DCA looks good.

[Copilot]

Copilot review overview

🔵 Needs a closer look

It changes foundational C# assignable, SSA, and local data-flow semantics that warrant final human validation.

Review effort: Balanced
Findings: None

What changed in this PR

Refactors C# pattern definitions and tuple-pattern flow to distinguish top-level matches from sub-pattern reads.

Changes:

• Adds TuplePatternExpr and limits direct flow to top-level tuple patterns.
• Represents nested pattern declarations as PatternDefinition.
• Updates dependent analyses and generated test expectations.

File	Description
Tuples.cs	Normalizes the final line.
tuples/​PrintAst.expected	Updates tuple-pattern AST expectations.
DataFlowStep.expected	Removes unwanted pattern-flow edges.
csharp8/​PrintAst.expected	Updates tuple-pattern classifications.
DeadStoreOfLocal.ql	Retains top-level pattern semantics.
Expr.qll	Adds TuplePatternExpr.
Nullness.qll	Restricts non-null definitions to top-level patterns.
DataFlowPrivate.qll	Refines tuple-pattern flow and read steps.
Assignable.qll	Extends pattern definitions to nested declarations.

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