Add note about reverting a workaround in the future

The root cause was fixed upstream in LLVM main. This adds a reminder to revert the workaround once the LLVM rustc depends on is new enough. Since I'm not sure how such optimizations get routed to LLVM releases, I used the conservative assumption that it will only show up with LLVM 13.

Author: LingMan

core/src/cmp.rs

@@ -982,6 +982,9 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn le(&self, other: &Rhs) -> bool {
         // Pattern `Some(Less | Eq)` optimizes worse than negating `None | Some(Greater)`.
+        // FIXME: The root cause was fixed upstream in LLVM with:
+        // https://github.com/llvm/llvm-project/commit/9bad7de9a3fb844f1ca2965f35d0c2a3d1e11775
+        // Revert this workaround once support for LLVM 12 gets dropped.
         !matches!(self.partial_cmp(other), None | Some(Greater))
     }