[ghost] Tests to support a \ghost qualifier

tests/syntax/ghost_cv_discarding.i

tests/syntax/ghost_cv_invalid_use.i

tests/syntax/ghost_cv_parsing_errors.c

tests/syntax/ghost_cv_valid_ref.i

tests/syntax/ghost_cv_valid_use.i

tests/syntax/ghost_cv_var_decl.c

tests/syntax/ghost_cv_var_decl.ml

tests/syntax/oracle/ghost_cv_discarding.res.oracle

tests/syntax/oracle/ghost_cv_invalid_use.res.oracle

tests/syntax/oracle/ghost_cv_parsing_errors.0.res.oracle

tests/syntax/oracle/ghost_cv_parsing_errors.1.res.oracle

tests/syntax/oracle/ghost_cv_parsing_errors.2.res.oracle

tests/syntax/oracle/ghost_cv_valid_ref.res.oracle

tests/syntax/oracle/ghost_cv_valid_use.res.oracle

tests/syntax/oracle/ghost_cv_var_decl.0.res.oracle

tests/syntax/oracle/ghost_cv_var_decl.1.res.oracle