parse.y: no dup US-ASCII string

* parse.y (rb_str_dynamic_intern): no needs to associate encoding
  if it is US-ASCII.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45430 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
parse.y: dup iff needed

* parse.y (rb_str_dynamic_intern): dup the given string only if
  needed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45431 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
parse.y: guard dup string

* parse.y (rb_str_dynamic_intern): prevent a string which may be
  duplicated, from GC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45432 b2dd03c8-39d4-4d8f-98ff-823fe69b080e