ruby27: Fix rescue parsing in multi assignement

[Morriar]

Motivation

As described in Ruby's change log, this PR changes how ruby27 parses the rescue modifier with multiple assignment to make it consistent with the singular form:

a = raise rescue 1
# => 1
a # => 1 in Ruby 2.6 and 2.7

a, b = raise rescue [1, 2]
# => [1, 2]

# 2.6
a # => nil
b # => nil
# The statement parsed as: (a, b = raise) rescue [1, 2]

# 2.7
a # => 1
b # => 2
# The statement parsed as: a, b = (raise rescue [1, 2])

This commit tracks upstream commit ruby/ruby@53b3be5.

Test plan

See included automated tests.

[jez]

Thanks!