Wrap include of <future> in some warning suppression pragmas

Eventually we may need to sink this include to the .cpp file or
something to suport LLVM_ENABLE_THREADS=OFF, but this solves my
immediate problem of fixing the build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255682 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed