gn build: Stop defining LLVM_ON_UNIX globally.

This macro is already being defined in llvm-config.h.

Differential Revision: https://reviews.llvm.org/D56626

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351154 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed