[LLVMdev] Metadata/Value split has landed

[Duncan P. N. Exon Smith]

The `Metadata`/`Value` split (PR21532) landed in r223802 -- at least, the
C++ side of it.  This was a rocky day, but I suppose that's what I get
for failing to stage the change in smaller pieces.

As of r223916 (lldb), I'm not aware of any remaining (in-tree) breakage,
so if I've missed some problem in the sea of buildbot errors, please
flag me down.

I'll follow up soon with bitcode and assembly changes!