Don't #include MemoryBuffer.h from Host.h.

It turns out this #include isn't used from Host.h anyway,
but by having it it causes circular include dependencies.
This issues only surfaced while I was working on a separate
patch, so I'm submitting this first so that it's independent
of the other, unrelated patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@318489 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed