Keep doc comment before declaration.

Because the attribute is part of the declaration, putting a doc comment
between the attribute and "public" ends up with the comment in the
middle of the declaration.  This results in SourceKit skipping over the
comment, and the docs not being shown.

Fixes <rdar://problem/58716408>.

Author: amartini51

stdlib/public/core/KeyPath.swift

@@ -30,10 +30,10 @@ internal func _abstract(
 // The two must coexist, so it was renamed. The old name must not be
 // used in the new runtime. _TtCs11_AnyKeyPath is the mangled name for
 // Swift._AnyKeyPath.
-@_objcRuntimeName(_TtCs11_AnyKeyPath)
 
 /// A type-erased key path, from any root type to any resulting value
 /// type.
+@_objcRuntimeName(_TtCs11_AnyKeyPath)
 public class AnyKeyPath: Hashable, _AppendKeyPath {
   /// The root type for this key path.
   @inlinable