[x86/SLH] Add the design document for Speculative Load Hardening,

[x86/SLH] Add the design document for Speculative Load Hardening,
a Spectre v1 mitigation.

This was initially posted w/ the patch implementing this, got some basic
review there. Also, it is generated from a the Google doc that I shared
as part of the Speculative Load Hardening RFC and which has seen pretty
widespread review at this point.

However, as the patches are landing in LLVM, I wanted to land the docs
as well. But it seemed like a bad idea to have them in the same commit
in case of reverts or other things. So the docs are split out here.

Thanks for all the review so far, and further review and improvements to
the documentation here welcome. Please feel free to keep hammering on
the code review or Google document.

Note that this is a markdown document which Sphinx doesn't yet process.
But we can add support for that after and this should get picked up
(and I'm preparing patches for that). Also, this gets the document
itself into a nice shared place where we can iterate on it.

Differential Revision: https://reviews.llvm.org/D49433

llvm-svn: 337391