unusedIdentifierFixes: Factor out nested switch statements and fix implicit fallthrough

[ghost]

Fixes #14885

Changes:

• Move all inner switch statements to their own functions.

• ForOfStatement handler now asserts the kind of the initializer instead of testing. Previous code would have fallen through and then crashed after casting the VariableDeclaration to a TypeParameter.

• Removed the handler for CatchClause from the VariableDeclaration handlers.

  • Looks like this was dead code.
  • It wouldn't compile any more now that we are typing varDecl: ts.VariableDeclaration correctly.
  • Caught exceptions aren't marked as unused variables.
  • An identifier is required in a catch clause.