Comment out unused macro because of warning

Modern compilers (Clang 3.4, GCC 4.8) warn on variadic macros being
introduced in C99, which produces a huge number of useless diagnostics
since this macro is unused in the whole project.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200479 91177308-0d34-0410-b5e6-96231b3b80d8