Add LLVM_HAS_INITIALIZER_LISTS for upcoming C++11 support.  Use it in ArrayRef

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194362 91177308-0d34-0410-b5e6-96231b3b80d8
Don't universally enable initialiser lists on GCC.  Thanks for catching this Chandler

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194365 91177308-0d34-0410-b5e6-96231b3b80d8