[PATCH] IR: Make metadata typeless in assembly

[Duncan P. N. Exon Smith]

Now that `Metadata` is typeless, reflect that in the assembly.

There's really only one reasonably thing to do here: strip the
`metadata` type (except in arguments to `call` instructions, where it
still has meaning), and add the symmetric `value` keyword for all
metadata operands that bridge to `Value`.

I'm confident in the code changes (although obviously I'll relish any
criticism offered), but I'd love a review of `docs/LangRef.rst`.  I
wasn't really sure how much exposition was appropriate, so I kept it
pretty simple.

Also, I wrote an upgrade script that handle all of the tests in llvm
after applying the main patch.  It even handles much of cfe (doing
mostly the right thing with `CHECK` lines).  I'll be attaching it the PR
after I make my commit, but feel free to spot-check its enormous result,
which I've attached as llvm-tests.patch.  (I'll squash that in with the
code changes when I commit.)

Note: in case it's confusing, some of the testcase changes in the code
patch are killing newlines so the upgrade script can do it's magic.

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