Now that -Wc++2a-compat is no longer enabled (bug 1887580), we don't need these pragmas disabling -Wc++2a-compat warnings about u8"" string literals' type changing from const char[] in C++17 to const char8_t[] in C++20.
Depends on D206742
Differential Revision: https://phabricator.services.mozilla.com/D206743