Useful for sanitizer fuzzer builds.
clang doesn't have a -fconcepts switch (I'm guessing it just enables
concepts automatically with -std=c++2a, but I haven't checked),
and at least the version on my system doesn't understand
-Wno-deprecated-move, so pass these two flags only to gcc.
In return, disable -Woverloaded-virtual which fires in many places.
The preceding commits fixed the handful of -Wunused-private-field
warnings that clang emitted.
a619943001
