make buildworld: "[...] fatal error: 'llvm/Demangle/Demangle.h' file not found", among other errors

I've tried re-cloning my source tree and am still getting this error when trying to build the world, among others.
The full error message is
--- Demangle/ItaniumDemangle.o ---
/usr/src/contrib/llvm-project/llvm/lib/Demangle/ItaniumDemangle.cpp:13:10: fatal error: 'llvm/Demangle/Demangle.h' file not found

Then, I also get these other ones when building with 16 parallel jobs:
--- Support/ABIBreak.o ---
/usr/src/contrib/llvm-project/llvm/lib/Support/ABIBreak.cpp:9:10: fatal error: 'llvm/Config/abi-breaking.h' file not found
[...]
--- Support/APFloat.o ---
/usr/src/contrib/llvm-project/llvm/lib/Support/APFloat.cpp:14:10: fatal error: 'llvm/ADT/APFloat.h' file not found
[...]
--- main.o ---
/usr/src/usr.sbin/config/main.cc:49:10: fatal error: 'y.tab.h' file not found
[...]

Is my copy broken, or is there something actually missing on the original repository?