Found two identical files named 'DefaultOnly.h' in the libcxx test suite. Moved one to /support, removed the other, and iupdated all the includes. No functionality change

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@196118 91177308-0d34-0410-b5e6-96231b3b80d8