Annotate some newly added unsafe expressions

Author: meg-gupta

stdlib/public/core/Span/MutableSpan.swift

@@ -182,7 +182,7 @@ extension MutableSpan where Element: BitwiseCopyable {
     @lifetime(self: copy self)
     set {
       _checkIndex(position)
-      self[unchecked: position] = newValue
+      unsafe self[unchecked: position] = newValue
     }
   }
 
@@ -197,7 +197,7 @@ extension MutableSpan where Element: BitwiseCopyable {
     @lifetime(self: copy self)
     set {
       let elementOffset = position &* MemoryLayout<Element>.stride
-      _start().storeBytes(of: newValue, toByteOffset: elementOffset, as: Element.self)
+      unsafe _start().storeBytes(of: newValue, toByteOffset: elementOffset, as: Element.self)
     }
   }
 }