[ADT][Expensive checks] Create a std::random_device seed only once when shuffling before sorting

This speeds up the build of compiler-rt with an expensive checks enabled clang by an order of
1 or 2 magnitudes on my machine. I was hoping this would also fix the 'large.test' libFuzzer
timeout on the expensive checks bot on green dragon http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-expensive/,
but the fuzzer test still takes too long to compile because of other IR/MIR verification inefficiencies.

Differential Revision: https://reviews.llvm.org/D70288