[Support] Reduce Dependence on Host.h

The intention behind this commit is to reduce the use of Host.h/Host.cpp
in Support, to where it is only necessary.

In this case, the endian-detection and support functionality needed by
these implementations can be provided by `Support/SwapByteOrder.h` in a
cleaner manner.

This patch also changes the byte swap in SHA256.cpp to use the byte swap
function from that header, rather than an inlined implementation.

Differential Revision: https://reviews.llvm.org/D137834