[hack] Fix grammar of switch statement
[hack] Fix grammar of switch statement

Summary:
The grammar for switch statements is simple and convenient for the parser author, but inconvenient for tools which need to process switch statements.

I propose a change to the specification and implementation of the switch AST to better match our intuitions about what a switch is.

Reviewed By: oulgen

Differential Revision: D4216559

fbshipit-source-id: 8968285ad528c4646ee3aa123313a91f35f90916

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