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
Zhihao Yuan noted that a move assignment operation was missing from std::adjacent_difference.  Fixed.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189036 91177308-0d34-0410-b5e6-96231b3b80d8
Zhihao Yuan noted that there were a few unneeded  statements.  Eliminated the unnecessary ones, and commented the ones that are there for non-obvious reasons such as to help things limp along in C++03 language mode.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189039 91177308-0d34-0410-b5e6-96231b3b80d8
XFAIL a few tests on darwin12

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189043 91177308-0d34-0410-b5e6-96231b3b80d8
Mark LWG issues #2083, 2099 and 2105 as complete. No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189044 91177308-0d34-0410-b5e6-96231b3b80d8