[PATCH] ValueMapper: Eliminate cross-file co-recursion, NFC

[Duncan P. N. Exon Smith via llvm-commits]

Eliminate co-recursion of Mapper::mapValue through
ValueMaterializer::materializeInitFor, through a major redesign of the
ValueMapper.cpp interface.

  - Expose a ValueMapper class that controls the entry points to the
    mapping algorithms.
  - Change IRLinker to use ValueMapper directly, rather than
    llvm::RemapInstruction, llvm::MapValue, etc.
  - Use (e.g.) ValueMapper::scheduleMapGlobalInit to add mapping work to
    a worklist in ValueMapper instead of recursing.

There were two fairly major complications.

Firstly, IRLinker::linkAppendingVarProto incorporates an on-the-fly IR
ugprade that I had to split apart.  Frankly I think this upgrade should
be done in the bitcode reader (and we should only accept the "new"
form), but for now I've just made it work and added a FIXME.

Secondly, IRLinker has special logic to correctly implement aliases with
comdats, and uses two ValueToValueMapTy instances and two
ValueMaterializers.  This wasn't hard to support by making ValueMapper
support arbitrary MappingContexts when scheduling work.

While out of scope for this commit, it should be straightforward to
remove recursion from Mapper::mapValue once this is in place.

(Note: 0001 is a prep commit that would be silly without 0002.)

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