cmake: update to LLVM 13 rc1

More precisely, aac4fe380d16a957627af2d6e5110ee35ad7e7e7 which is the
current tip of release/13.x.

Immediately following commits are tracking the same LLVM version.
78ff2a148aAndrew Kelley committed on 8/16/2021, 12:48:37 AMparente9bf801
3 files changedLine totals unavailable