[libcxx] Fix test failure on GCC 4.9

GCC 4.9 seems to think that a constexpr default constructor implies
the constructor to be noexcept.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348850 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed