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

[Peter Collingbourne]

  I agree that it would be useful to have an independent mechanism for enabling these extensions (if only because it would allow people with compilers without a GCC-style driver to take advantage of them). But as for extending the use of constexpr beyond what libstdc++ does, I worry about the consequences of having yet another C++ standard library extension out there.

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