Work around errornous warning in gcc <= 12.2

gcc 12.[12] emits a -Wrestrict warning if compiled
with -O2/-O3 and -std=c++20 enabled when assigning
a string with a single character to a std::string.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105329

This is benign; disable this broken warning for up to 12.4.

PiperOrigin-RevId: 509358966