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)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171359 91177308-0d34-0410-b5e6-96231b3b80d8
Update the configure scripts as well as the makefiles for the move from
VMCore to IR.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171360 91177308-0d34-0410-b5e6-96231b3b80d8
Update the root Makefile for VMCore -> IR. Missed by my original search
sadly, and is somewhat surprising in general.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171361 91177308-0d34-0410-b5e6-96231b3b80d8