Add RemoveUnreachableMultiCatchAlternative

[steve-aom-elliott]

Summary

Adds RemoveUnreachableMultiCatchAlternative, which removes catch alternatives that are already covered by an earlier catch clause in the same try:

• catch (X) {} catch (X | Y) {} → catch (X) {} catch (Y) {}
• catch (X) {} catch (Y | X) {} → catch (X) {} catch (Y) {}
• catch (IOException) {} catch (FileNotFoundException | IllegalStateException) {} → catch (IOException) {} catch (IllegalStateException) {} (subtype shadowed)
• catch (IOException) {} catch (FileNotFoundException | EOFException) {} → catch (IOException) {} (whole later catch dropped)

When a multi-catch reduces to a single alternative it collapses back to a regular catch; when all alternatives are shadowed the entire catch clause is removed.

Motivation

The pattern shows up after type-substitution migrations that cause two catch clauses to overlap — for example, when a recipe renames an exception so its parent and child end up in adjacent catches. The result is a Java compile error (exception X has already been caught). This recipe cleans that up by dropping the unreachable alternative.

Intended to run before CombineSemanticallyEqualCatchBlocks so collapsed singletons become candidates for combination.

Notes

• Gated on NoMissingTypes — won't fire when type attribution is incomplete.
• No RSPEC tag attached: I couldn't find a Sonar rule that specifically targets cross-clause shadowing (Sonar likely treats it as a javac error). Happy to add one if reviewers know of a match.

Test plan

• dropAlternativeShadowedByEarlierCatch — catch (X) ... catch (X | Y) (@DocumentExample)
• dropTrailingAlternativeShadowedByEarlierCatch — catch (X) ... catch (Y | X)
• dropMiddleAlternativeShadowedByEarlierCatch — catch (X) ... catch (Y | X | Z)
• dropSubtypeAlternativeShadowedByEarlierCatch — alternative is a subtype
• dropEntireCatchWhenAllAlternativesAreShadowed
• dropEntireSingleTypeCatchWhenShadowed
• doNothingWhenOrderingMeansNothingIsShadowed
• doNothingWhenMultiCatchAlternativesAreAllNeeded

[knutwannheden]

When the recipe drops an entire catch clause (the fully-shadowed multi-catch and the shadowed single-type catch paths), the exception types' imports can be left behind unused. Calling maybeRemoveImport for the removed types would tidy that up. Minor — not a blocker.