Installing LLVM headers

[coypu%sdf.org@localhost]

Hi folks,

I am working on an update to Mesa 18.3.1 and using LLVM (we weren't
before).

As part of this, I'll need some LLVM headers. There's a lot of work
still to be done, but this is part 1 of several very large diffs to do
this.

This is the diff to do so:
http://coypu.sdf.org/llvm-headers.diff

Missing as far as I'm concerned (TODO):
/usr/include/llvm/Config/llvm-config.h

Didn't add, but I can do it if we do want it:
/usr/include/llvm/FuzzMutate/*
/usr/include/llvm/WindowsManifest/*
/usr/include/llvm/WindowsResource/*

Note: I don't intend to wait 2-4 weeks to commit this, I mostly wait for
joerg's complaints as he is the one that is maintaining llvm in tree.