Rename VMCore directory to IR.

Aside from moving the actual files, this patch only updates the build
system and the source file comments under lib/... that are relevant.

I'll be updating other docs and other files in smaller subsequnet
commits.

While I've tried to test this, but it is entirely possible that there
will still be some build system fallout.

Also, note that I've not changed the library name itself: libLLVMCore.a
is still the library name. I'd be interested in others' opinions about
whether we should rename this as well (I think we should, just not sure
what it might break)