Update the minimum external LLVM to 13

Author: cuviper

core/src/cmp.rs

@@ -1139,11 +1139,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     #[must_use]
     #[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))
+        matches!(self.partial_cmp(other), Some(Less | Equal))
     }
 
     /// This method tests greater than (for `self` and `other`) and is used by the `>` operator.