[PATCH] IR: Add specialized debug info metadata nodes

[Duncan P. N. Exon Smith]

Add specialized debug info metadata nodes that match the `DIDescriptor`
wrappers (used by `DIBuilder`) closely.  Assembly and bitcode support to
follow soon (it'll mostly just be obvious), but this sketches in today's
schema.  This is the first big commit (well, the only *big* one aside
from the testcase changes that'll come when I move this into place) for
PR22464.

I've marked a bunch of obvious changes as `TODO`s in the source; I plan
to make those changes promptly after this hierarchy is moved underneath
`DIDescriptor`, but for now I'm aiming mostly to match the status quo.

I don't need a low-level review here (although always welcome); I'm
mainly looking for the following:

  - The word 'context' is overloaded: `MDNode::getContext()` already
    exists, and returns an `LLVMContext&`; `DIDescriptor` uses 'context'
    to mean "the node that this one is defined inside".  I chose the
    word 'parent' instead of 'context' here.  Is this word okay?  If
    not, what about 'scope'?  This will be reflected in the assembly
    changes to come (I'd like the C++ names to match the assembly names,
    although technically it's not necessary).
  - Aside from the above, did I fail to match the current schema (in
    llvm/IR/DebugInfo.h) some other way?
  - I did this work as one big chunk because it was the easiest way for
    me to match the hierarchy.  I imagine reviewing it as a whole is
    easier for the same reason, but if someone has a strong preference
    for this to be broken up when I commit it (it'd take some work, but
    it would be mechanical) let me know.

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