Revert D121556 "[randstruct] Add randomize structure layout support"

This reverts commit 3f0587d0.

Not all tests pass after a few rounds of fixes.

I spot one failure that std::shuffle (potentially different results with
different STL implementations) was misused and replaced it with llvm::shuffle,
but there appears to be another failure in a Windows build.

The latest failure is reported on https://reviews.llvm.org/D121556#3440383