Parse rescue modifier values as statements

[kddnewton]

The value of a rescue modifier is a full stmt in the grammar in three of its six productions, but we parsed it as a plain expression in every case. This rejected valid syntax such as a rescue b, c = 1 and accepted invalid syntax such as a = 1 rescue not b.

These are the relevant grammar rules:

  stmt            : stmt modifier_rescue stmt
                  | mlhs '=' mrhs_arg modifier_rescue stmt
  command_rhs     : command_call_value modifier_rescue stmt
  arg_rhs         : arg modifier_rescue arg
  endless_arg     : endless_arg modifier_rescue arg
  endless_command : endless_command modifier_rescue arg

parse_rescue_modifier_value will now parse either a stmt or an arg depending on the caller.

Supersedes #4128