Silence g++ 4.9 build issue

lib/Support/ThreadLocal.cpp:53:15: error: typedef 'SIZE_TOO_BIG' locally defined but not used [-Werror=unused-local-typedefs]
   typedef int SIZE_TOO_BIG[sizeof(pthread_key_t) <= sizeof(data) ? 1 : -1];

Done the C++11 way, switching on and using LLVM_STATIC_ASSERT() instead of LLVM_ATTRIBUTE_UNUSED.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198255 91177308-0d34-0410-b5e6-96231b3b80d8