Unconditionally #include <future>

This unbreaks building with `LLVM_ENABLE_THREADS=0`. Since
https://github.com/llvm/llvm-project/commit/069919c9ba33 usage of
`std::promise` is not guarded by `LLVM_ENABLE_THREADS`, so this header
must be unconditionally included.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D89758