Synchronize submodules with LLVM at llvm/llvm-project@a7b1872593db

Updates LLVM dependencies to match
[a7b1872593db](https://github.com/llvm/llvm-project/commit/a7b1872593db).
- TensorFlow to
[908116d4231c](https://github.com/tensorflow/tensorflow/commit/908116d4231c)
- MLIR-HLO to
[08e1fdad829a](https://github.com/tensorflow/mlir-hlo/commit/${MLIR_HLO_SHA?})

`./scripts/git/update_to_llvm_syncpoint.py`

Automated submodule bump from .github/workflows/update_llvm_dependent_submodules.yml

PiperOrigin-RevId: 407872701