[libc++] Harden usage of static_assert against C++03
[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: 756ef344af921d95d562d9e9f9389127a89a6314@373328 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx