[libcxx] Use __decltype instead of __typeof__

Summary:
Both clang and GCC provide C++11 decltype semantics as __decltype in c++03 mode. We should use this instead of __typeof__ when availble. 

GCC added __decltype in 4.6.0, and AFAIK clang provided __decltype ever since 3.3. Unfortunately `__has_builtin(__decltype)` doesn't work for clang so we need to check the compiler version instead.


Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10426

llvm-svn: 239662