Mark test types for <atomic> nothrow default constructible. Patch from Steve MacKenzie.

The way the standard currently specifies the default constructor for atomic<T>
requires T to be nothrow default constructible. This patch makes our test types
meet this requirement.

Note: The nothrow default constructible requirment is subject to the outcome of
LWG issue 1265.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216561 91177308-0d34-0410-b5e6-96231b3b80d8