Integrate LLVM at d8b985c9 (iree-org#18061)

Advance to llvm/llvm-project@d8b985c9

Existing local patches carried over:
* Still carrying a revert of
llvm/llvm-project@f6431f0c

Dropped local patches:
* No longer carrying a revert of
llvm/llvm-project@fa066687
* No longer carrying a revert of
llvm/llvm-project@bbd4af5d

Signed-off-by: Benoit Jacob <jacob.benoit.1@gmail.com>