merge revision(s) 62743: [Backport #16687]

	Fix FrozenError on `Ripper.slice`

	Currently `Ripper.slice` raises a FrozenError

	```ruby
	require 'ripper'
	p Ripper.slice('foo', 'ident')
	```

	```
	/path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:193:in `concat': can't modify frozen String (FrozenError)
		from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:193:in `block in compile'
		from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:190:in `scan'
		from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:190:in `compile'
		from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:169:in `initialize'
		from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:151:in `new'
		from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:151:in `token_match'
		from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:144:in `slice'
		from /tmp/tmp.kb4cnhvum2/test.rb:2:in `<main>'
	```

	This patch will fix the problem.

	[Fix GH-1837]

	From: Masataka Pocke Kuwabara <kuwabara@pocke.me>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67871 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/ripper/lib/ripper/lexer.rb

@@ -192,7 +192,7 @@ def compile(pattern)
       if m = /[^\w\s$()\[\]{}?*+\.]/.match(pattern)
         raise CompileError, "invalid char in pattern: #{m[0].inspect}"
       end
-      buf = ''
+      buf = +''
       pattern.scan(/(?:\w+|\$\(|[()\[\]\{\}?*+\.]+)/) do |tok|
         case tok
         when /\w/

version.h

@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.5.8"
 #define RUBY_RELEASE_DATE "2020-03-31"
-#define RUBY_PATCHLEVEL 220
+#define RUBY_PATCHLEVEL 221
 
 #define RUBY_RELEASE_YEAR 2020
 #define RUBY_RELEASE_MONTH 3