David Blaikie suggested this test for detecting constexpr on the error_category default constructor.  Additionally I decided to make this constructor trivial as well, as I could not come up with a rationale for not doing so.  Fully tested with both current Apple clang and tip-of-trunk clang.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189034 91177308-0d34-0410-b5e6-96231b3b80d8