[stdlib] Add a couple of FIXME(ABI) notes

Author: Dave Abrahams

stdlib/public/core/Mirror.swift

@@ -385,6 +385,8 @@ public protocol CustomLeafReflectable : CustomReflectable {}
 ///
 /// Do not declare new conformances to this protocol; they will not
 /// work as expected.
+// FIXME(ABI): this protocol should be “non-open” and you shouldn't be able to
+// create conformances.
 public protocol MirrorPath {}
 extension IntMax : MirrorPath {}
 extension Int : MirrorPath {}

stdlib/public/core/Stride.swift.gyb

@@ -24,8 +24,6 @@
 /// Conforming types are notionally continuous, one-dimensional
 /// values that can be offset and measured.
 public protocol ${Self} : ${Conformance} {
-  // FIXME(ABI)(compiler limitation): We'd like to name this type "Distance"
-  // but for <rdar://problem/17619038>
   /// A type that can represent the distance between two values of `Self`.
   associatedtype Stride : SignedNumber