Integrate LLVM at `a86e966a` (#18644)

Existing reverts carried over:
1. Revert https://github.com/llvm/llvm-project/pull/100667)
- As noted by @hanhanW on https://github.com/iree-org/iree/pull/18619,
that PR "breaks the stablehlo build. We need to wait stablehlo bumping
LLVM ahead of it and fix the issue. Then we can bump stablehlo and drop
the local commit together."
2. Revert https://github.com/llvm/llvm-project/pull/110170)
   - That is just the Bazel change accompanying 1.

Signed-off-by: Benoit Jacob <jacob.benoit.1@gmail.com>
diff --git a/third_party/llvm-project b/third_party/llvm-project
index aca2758..acaf557 160000
--- a/third_party/llvm-project
+++ b/third_party/llvm-project
@@ -1 +1 @@
-Subproject commit aca2758413c151d3aa3518a3f99c4ab7ec76d876
+Subproject commit acaf557f49a53228b5869c878e857e8fbb69d5ce