[libcxx] Fix usage of _C2, which is a "nasty macro" in some environments

The problem was pointed out in https://reviews.llvm.org/D48896#inline-475775.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345834 91177308-0d34-0410-b5e6-96231b3b80d8