[NFC][libcxx] Add comments about making mutex/condition_variable trivial on Apple platforms

Leaving some comments behind so that we avoid re-having that discussion
in the future.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@367048 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed