[PATCH] [libc++] In C++11 GNU extension mode, start emulating libstdc++'s use of constexpr.

[Peter Collingbourne]

Hi rsmith, howard.hinnant,

libstdc++ always declares various entities as constexpr if C++14 makes
them constexpr, even in C++11 mode. To support programs which rely on this
behavior, start applying constexpr to those entities in C++11 mode only if
GNU extensions are enabled (i.e. -std=gnu++11).

We have to be selective about which entities we do this for because of the
relaxed constexpr rules in C++14.

This patch applies the change to entities in <utility>, as well as its
dependencies, std::move and std::forward.

http://llvm-reviews.chandlerc.com/D2276

Files:
  include/__config
  include/__tuple
  include/type_traits
  include/utility
  test/utilities/utility/pairs/pair.astuple/get_const.pass.cpp
  test/utilities/utility/pairs/pair.astuple/get_non_const.pass.cpp
  test/utilities/utility/pairs/pair.astuple/pairs.by.type1.fail.cpp
  test/utilities/utility/pairs/pair.astuple/pairs.by.type2.fail.cpp
  test/utilities/utility/pairs/pair.astuple/pairs.by.type3.fail.cpp
  test/utilities/utility/pairs/pairs.pair/const_first_const_second.pass.cpp
  test/utilities/utility/pairs/pairs.pair/const_pair_U_V.pass.cpp
  test/utilities/utility/pairs/pairs.pair/copy_ctor.pass.cpp
  test/utilities/utility/pairs/pairs.pair/default.pass.cpp
  test/utilities/utility/pairs/pairs.spec/comparison.pass.cpp
  test/utilities/utility/pairs/pairs.spec/make_pair.pass.cpp
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