Move Predicate.h from Host to Utility

Summary:
This class was initially in Host because its implementation used to be
very OS-specific. However, with C++11, it has become a very simple
std::condition_variable wrapper, with no host-specific code.

It is also a general purpose utility class, so it makes sense for it to
live in a place where it can be used by everyone.

This has no effect on the layering right now, but it enables me to later
move the Listener+Broadcaster+Event combo to a lower layer, which is
important, as these are used in a lot of places (notably for launching a
process in Host code).

Reviewers: jingham, zturner, teemperor

Reviewed By: zturner

Subscribers: xiaobai, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D50384

llvm-svn: 341089