IR: Add distinct MDNodes as a first-class concept

[Duncan P. N. Exon Smith]

In order to "defeat" `MDNode` uniquing, metadata schemas resort to
using self-references to prevent merging.

Introduce a `distinct` designator which explicitly requests this
behaviour, without the need for a self-reference.  These nodes are
still stored in the `LLVMContext`, but are not uniqued based on
their operands (well, not uniqued at all).

  - This concept *already exists*.  Before the metadata/value split,
    it was used whenever operands went to null (to prevent teardown
    madness, but it happened more frequently).  It's still in use
    for self-referencing `MDNode`s, as well as when
    `MDNode::replaceOperandWith()` causes a uniquing collision (see
    module flags behaviour before r225397 for an example).

    Some recent commits have already exposed `getDistinct()` and
    `isDistinct()` as API (see r225401 and r225406).

  - The first patch adds assembly/bitcode support.  I chose the
    'distinct' keyword.  It has an accompanying clang patch to
    update testcases.

  - The second patch adds support to `MapMetadata()` to maintain the
    'distinct'-ness.

  - The third patch just clears out the TODO.

I haven't updated any metadata schemas here; I figure the owners of
the schemas can update those in their own time.

My main concern here was about how to deal with `MapMetadata()` --
it feels wrong to just duplicate everything.  However, I looked
through all the callers.  In *most* cases RF_NoModuleLevelChanges
is passed, and in the others this naive solution seems correct (or
at least harmless).

If this becomes an issue (i.e., there's a place with module-level
changes where distinct metadata should only be duplicated when
operands change), we can add another flag to decide whether to
duplicate it.  For now, this just matches the behaviour we already
get out of self-references.

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