[base] Remove special-casing for gcc < 5

Node now requires gcc >=6.3, and we do not test on gcc <5.4 any more.
Thus remove a special case for gcc <5.

R=machenbach@chromium.org

Bug: v8:9686
Change-Id: Ifffddec611c15b704aa292a65e87cd770d85ea7b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1786283Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63572}

src/base/macros.h

@@ -232,35 +232,16 @@ struct is_trivially_copyable {
       // the standard does not, so let's skip this check.)
       // Trivial non-deleted destructor.
       std::is_trivially_destructible<T>::value;
-
-#elif defined(__GNUC__) && __GNUC__ < 5
-  // WARNING:
-  // On older libstdc++ versions, there is no way to correctly implement
-  // is_trivially_copyable. The workaround below is an approximation (neither
-  // over- nor underapproximation). E.g. it wrongly returns true if the move
-  // constructor is non-trivial, and it wrongly returns false if the copy
-  // constructor is deleted, but copy assignment is trivial.
-  // TODO(rongjie) Remove this workaround once we require gcc >= 5.0
-  static constexpr bool value =
-      __has_trivial_copy(T) && __has_trivial_destructor(T);
-
 #else
   static constexpr bool value = std::is_trivially_copyable<T>::value;
 #endif
 };
-#if defined(__GNUC__) && __GNUC__ < 5
-// On older libstdc++ versions, base::is_trivially_copyable<T>::value is only an
-// approximation (see above), so make ASSERT_{NOT_,}TRIVIALLY_COPYABLE a noop.
-#define ASSERT_TRIVIALLY_COPYABLE(T) static_assert(true, "check disabled")
-#define ASSERT_NOT_TRIVIALLY_COPYABLE(T) static_assert(true, "check disabled")
-#else
 #define ASSERT_TRIVIALLY_COPYABLE(T)                         \
   static_assert(::v8::base::is_trivially_copyable<T>::value, \
                 #T " should be trivially copyable")
 #define ASSERT_NOT_TRIVIALLY_COPYABLE(T)                      \
   static_assert(!::v8::base::is_trivially_copyable<T>::value, \
                 #T " should not be trivially copyable")
-#endif
 
 // The USE(x, ...) template is used to silence C++ compiler warnings
 // issued for (yet) unused variables (typically parameters).