IR: Use an enum to describe Metadata storage, NFC

More clearly describe the type of storage used for `Metadata`.

  - `Uniqued`: uniqued, stored in the context.
  - `Distinct`: distinct, stored in the context.
  - `Temporary`: not owned by anyone.

This is the first in a series of commits to fix a design problem with
`MDNodeFwdDecl` that I need to solve for PR22235.  While `MDNodeFwdDecl`
works well as a forward declaration, we use `MDNode::getTemporary()` for
more than forward declarations -- we also need to create early versions
of nodes (with fields not filled in) that we'll fill out later (see
`DIBuilder::finalize()` and `CGDebugInfo::finalize()` for examples).
This was a blind spot I had when I introduced `MDNodeFwdDecl` (which
David Blaikie (indirectly) highlighted in an unrelated review [1]).

[1]: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150112/252381.html

In general, we need `MDTuple::getTemporary()` to give a temporary tuple
(like `MDNodeFwdDecl`), `MDLocation::getTemporary()` to give a temporary
location, and (the problem at hand) `GenericDebugMDNode::getTemporary()`
to give a temporary generic debug node.

So I need to fold the idea of "temporary" nodes back into
`UniquableMDNode`.  (More commits to follow as I refactor.)

llvm-svn: 226481