Strings concatenated with backslash may end up being frozen when they
shouldn't be.  This commit fixes the issue.  It required a change
upstream in Prism, but also a change to the Prism compiler in CRuby.

  https://github.com/ruby/prism/pull/3606

[Bug #21187]

This reverts commit a495e6a44ce8cff17461b250e32ab63e409a642d.

This break extension builds:

```
/Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:321:in 'String#replace': can't modify frozen String: "$(SDKROOT)$(prefix)/include" (FrozenError)
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:321:in 'RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:325:in 'block in <module:RbConfig>'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:324:in 'Hash#each_value'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:324:in '<module:RbConfig>'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:11:in '<top (required)>'
        from ./ext/extmk.rb:42:in 'Kernel#require'
        from ./ext/extmk.rb:42:in '<main>'
make[1]: *** [ext/configure-ext.mk:70: ext/json/exts.mk] Error 1
```

https://github.com/ruby/prism/commit/6f886be0a4