[Bug bootstrap/64389] duplicate symbols in libstdc++.6.dylib on darwin due to r219012

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64389

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I committed a different fix, which gets rid of the TODO in string-inst.cc, let
me know if it still doesn't bootstrap!