Build failure with LLVM git - elementtype attribute missing

[LebedevRI]

$ ninja -j1
[  1% 45/3183][  1% 0:00:00 + 0:00:08] Generating initmod.hvx_128.bc
FAILED: src/runtime/initmod.hvx_128.bc /builddirs/halide/build-Clang-Release-LLVMgit/src/runtime/initmod.hvx_128.bc 
cd /builddirs/halide/build-Clang-Release-LLVMgit/src/runtime && /builddirs/llvm-project/build-Clang13/bin/llvm-as /repositories/halide/src/runtime/hvx_128.ll -o initmod.hvx_128.bc
/builddirs/llvm-project/build-Clang13/bin/llvm-as: assembly parsed, but does not verify as correct!
Operand for indirect constraint must have elementtype attribute
  call void asm sideeffect "vmem($0 + #0):scatter_release\0A; v1 = vmem($0 + #0)\0A", "=*m,*m,~{v1}"(i8* %ptr, i8* %ptr)
ninja: build stopped: subcommand failed.

While the fix is straight-forward, said attribute only appeared in LLVM13, and LLVM12 (which is the oldest supported)
barfs on it: https://godbolt.org/z/EK6cb37n5

[steven-johnson]

LLVM12 barfs on it

Ouch, that's going to make things slightly less pleasant, as we'll have to fork the files. Oh well.