[atomics] Fix standard-layout requirement for atomic types (#1021)

Fixes #506.

Author: jensmaurer

source/atomics.tex

@@ -605,13 +605,14 @@
 
 \pnum
 The atomic integral specializations and the specialization \tcode{atomic<bool>}
-shall have standard layout. They shall each have a trivial default constructor
+shall be standard-layout structs. They shall each have a trivial default constructor
 and a trivial destructor. They shall each support aggregate initialization
 syntax.
 
 \pnum
 There shall be pointer partial specializations of the \tcode{atomic} class template.
-These specializations shall have standard layout, trivial default constructors, and trivial destructors.
+These specializations shall be standard-layout structs.
+They shall each have a trivial default constructor and a trivial destructor.
 They shall each support aggregate initialization syntax.
 
 \pnum
@@ -1259,7 +1260,8 @@
 \tcode{atomic_flag}, though with less than ideal properties. \end{note}
 
 \pnum
-The \tcode{atomic_flag} type shall have standard layout. It shall have a trivial default constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor.
+The \tcode{atomic_flag} type shall be a standard-layout struct.
+It shall have a trivial default constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor.
 
 \pnum
 The macro \tcode{ATOMIC_FLAG_INIT} shall be defined in such a way that it can be used to initialize an object of type \tcode{atomic_flag} to the