Emit correct IL for exhaustive switch statements that use a separate Dag for lowering

[AlekseyTs]

Fixes #85809

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Copilot review overview

🔵 Needs a closer look

It changes compiler control-flow and IL generation, and the full CI suite is still running.

Review effort: Balanced
Findings: 2

Open (2)

• Fix merged words in explanatory comments · New
• Synchronize expected diagnostic with unreachable1 source text · New

What changed in this PR

Fixes invalid IL from exhaustive switch statements when lowering requires a separate decision DAG.

Changes:

• Emits an unreachable-path exception instead of falling through.
• Reuses switch-expression exception selection logic.
• Adds extensive union and list-pattern IL tests.

Minor comment/documentation corrections were identified.

File	Description
CSharpTestBase.cs	Updates DAG helper call.
PatternMatchingTests_ListPatterns.cs	Adds list-pattern regression coverage.
UnionsTests.cs	Adds union switch coverage.
SynthesizedThrowSwitchExpressionExceptionMethod.cs	Extracts throw generation.
SynthesizedParameterlessThrowMethod.cs	Extracts parameterless throw generation.
LocalRewriter_SwitchExpression.cs	Shares exception-selection logic.
LocalRewriter_PatternSwitchStatement.cs	Emits the fallback throw path.
LocalRewriter_BasePatternSwitchLocalRewriter.cs	Centralizes exception strategy selection.
BoundSwitchStatement.cs	Tracks lowering-only default labels.

[Copilot]

Copilot review overview

🔵 Needs a closer look

It changes compiler control flow and emitted IL, and the latest CI run remains in progress.

Review effort: Balanced
Findings: 1

Open (1)

• Fix merged words in explanatory comments

Resolved since last review (1)

• Synchronize expected diagnostic with unreachable1 source text

Previously missed (1)

In code that hasn't changed since last review

Comment incorrectly says unmatched input uses BreakLabel

src/​Compilers/​CSharp/​Portable/​BoundTree/​BoundSwitchStatement.cs:32

This branch assigns the declared default label, not BreakLabel; the comment currently misstates the destination for unmatched input.

[AlekseyTs]

@RikkiGibson, @jjonescz, @dotnet/roslyn-compiler Please review

[AlekseyTs]

@RikkiGibson, @jjonescz For a second review