ANTLR backend: start rules needed

[andreasabel]

See #163 and antlr/antlr4#2689 : ANTLR will sometimes require a start rule

  StartX : X EOF;

for entrypoint X. The current ANTLR backend does not emit such start rules (the LR parser generators add it automatically), thus, the parser fails in some cases.
TODO: emit such start rules when generating the ANTLR input.

[andreasabel]

BNFC now emits start rules for all entrypoints in the ANTLR backend.