Avoid embedded preprocessor directives in __tree

Similar to rL242623, move C++ version checks outside of _NOEXCEPT_()
macro invocation argument lists, to avoid "embedding a directive within
macro arguments has undefined behavior" warnings.

Differential Revision: https://reviews.llvm.org/D23961


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@279926 91177308-0d34-0410-b5e6-96231b3b80d8