Integrate LLVM at llvm/llvm-project@f2e6ca805dbb (#21805)

Carries the revert of
https://github.com/llvm/llvm-project/commit/b4c31dc98dfc929728904cd96f0f4cf812c4d5b5

No changes to IREE required.

Signed-off-by: Ian Wood <ianwood@u.northwestern.edu>
diff --git a/third_party/llvm-project b/third_party/llvm-project
index c216fe7..978731e 160000
--- a/third_party/llvm-project
+++ b/third_party/llvm-project
@@ -1 +1 @@
-Subproject commit c216fe704a1c0ef721e1aebe138ff211cb2e9375
+Subproject commit 978731ed18103b3fed88f7933a6da08fc1044e6f