Define LLVM_HAS_STRONG_ENUMS

This is needed to guard an upcoming feature in clang until the C++11 transition
is complete, at which point it can be removed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197895 91177308-0d34-0410-b5e6-96231b3b80d8