Fix comment to more accurately describe C++ language requirements around tail padding.

[rsmith]

Summary

As of C++ core issue 43 (http://wg21.link/cwg43), which was voted into
the C++ working draft in 1999, it is not permissible to memcpy a base
class subobject, even if it's of POD type, so there is no problem with
reusing the tail padding of a base class. That issue was voted into the
standard in DR status, so it applies retroactively to C++98 (and is in
any case part of C++03).

So stop suggesting that AlwaysUseTailPadding mode is non-conforming.

[rjmccall]

LGTM

clang/include/clang/Basic/TargetCXXABI.h

bool canKeyFunctionBeInline() const {
     }
     llvm_unreachable("bad ABI kind");
   }
 
   /// When is record layout allowed to allocate objects in the tail
   /// padding of a base class?
   ///
   /// This decision cannot be changed without breaking platform ABI
-  /// compatibility, and yet it is tied to language guarantees which
-  /// the committee has so far seen fit to strengthen no less than
-  /// three separate times:
-  ///   - originally, there were no restrictions at all;
-  ///   - C++98 declared that objects could not be allocated in the
-  ///     tail padding of a POD type;
-  ///   - C++03 extended the definition of POD to include classes
-  ///     containing member pointers; and
-  ///   - C++11 greatly broadened the definition of POD to include
-  ///     all trivial standard-layout classes.
-  /// Each of these changes technically took several existing
-  /// platforms and made them permanently non-conformant.
+  /// compatibility. In ISO C++98, tail padding reuse was only permitted for
+  /// non-POD base classes, but that restriction was removed retroactively by
+  /// DR 43, and tail padding reuse is always permitted in all de facto C++
+  /// language modes. However, many platforms use a variant of the old C++98
+  /// rule for compatibility.
   enum TailPaddingUseRules {
     /// The tail-padding of a base class is always theoretically
-    /// available, even if it's POD.  This is not strictly conforming
-    /// in any language mode.
+    /// available, even if it's POD.
     AlwaysUseTailPadding,
 
     /// Only allocate objects in the tail padding of a base class if
     /// the base class is not POD according to the rules of C++ TR1.
-    /// This is non-strictly conforming in C++11 mode.
     UseTailPaddingUnlessPOD03,
 
     /// Only allocate objects in the tail padding of a base class if
     /// the base class is not POD according to the rules of C++11.
     UseTailPaddingUnlessPOD11
   };
   TailPaddingUseRules getTailPaddingUseRules() const {
     switch (getKind()) {