This fixes https://bugs.ruby-lang.org/issues/18038. The provided
reproduction showed that this happens in heredocs with double
interpolation. In this case `DSTR` was getting returned but needs to be
convered to a `EVSTR` which is what is returned by the function. There
may be an additional bug here that we weren't able to produce. It seems
odd that `STR` returns `DSTR` while everything else should return
`EVSTR` since the function is `new_evstr`.
[Bug #18038][ruby-core:104597]
Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>
b940a45357