[libc++] Harden usage of static_assert against C++03

In C++03, we emulate static_assert with a macro, and we must parenthesize
multiple arguments.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373328 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed