* error.c: add Exception#backtrace_locations.
  Now, there are no setter and independent from Exception#backtrace.
  [Feature #8960]
* eval.c (setup_exception): set backtrace locations for `bt_location'
  special attribute.
* vm_backtrace.c (rb_backtrace_to_location_ary): added.
* internal.h: ditto.
* test/ruby/test_backtrace.rb: add a test for
  Exception#backtrace_locations.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44170 b2dd03c8-39d4-4d8f-98ff-823fe69b080e