[libcxx][NFC] Properly indent nested #ifdefs and #defines

I just realized I had always been reading this wrong because of the lack
of indentation, so I'm re-indenting this properly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349408 91177308-0d34-0410-b5e6-96231b3b80d8