[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.
llvm-svn: 373328
This is a follow-up to r354460, which causes errors for pre-C++11
programs using <cmath>, similar to:
/usr/include/c++/v1/cmath:622:68: error: too many arguments provided to
function-like macro invocation
Reported by: antoine
MFC after: immediately (because of ports breakage)
a600a25e3e