tests: Remove useless autoextern test cases

Since multiple input files are now concatenated at CPP time, there's no
difference as far as C parsing or AST work (such as adding the Extern
block) is concerned.
ast: Remove astOpsC2FB() and handle it during emitting
ast: Change node debug ids to match the ASTCLASS_* name exactly
emit: Split emitAst() into emitCode() and emitExpr()