Adopt c++20's <bit> functions
Created attachment 450544 [details] Patch
Probably, this needs to wait for a bit more period since currently GCC is 8.3, which does not support <bit>. At April, we can bump it to 9. https://trac.webkit.org/wiki/WebKitGTK/GCCRequirement
Ugh, looks like the 10.15 builders are also unhappy. Bummer.