[randstruct] Use llvm::shuffle to avoid STL impl difference after D121556

This reverts commit 2a2149c754f96376ddf8fed248102dd8e6092a22.
This reverts commit 8d7595be1dd41d7f7470ec90867936ca5e4e0d82.
This reverts commit e2e6899452998932b37f0fa9e66d104a02abe3e5.

If this doesn't work, I'll revert the whole thing.

GitOrigin-RevId: 46b2a463bdef1bd1d80abee869b09f95ca5a4fc2
3 files changed