Replace 'noexcept' with '_NOEXCEPT' in <shared_mutex>. This allows us to build the dylib with MSVC, which doesn't support noexcept (sheesh\!). Thanks to K-ballo for the report.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216384 91177308-0d34-0410-b5e6-96231b3b80d8