[Bug tree-optimization/105651] [12/13 Regression] bogus "may overlap" memcpy warning with std::string and operator+ at -O3

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105651

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think the vast majority of string mutations involve
inserting/appending/substituting an unrelated string, so non-overlapping. We
can make your change on trunk.