Bump externals/llvm-project from aec0fbb to ac378c2

Bumps [externals/llvm-project](https://github.com/Xilinx/llvm-project) from `aec0fbb` to `ac378c2`.
- [Commits](Xilinx/llvm-project@aec0fbb...ac378c2)

---
updated-dependencies:
- dependency-name: externals/llvm-project
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>