IR: Lazily create ReplaceableMetadataImpl on MDNode

RAUW support on MDNode usually requires an extra allocation for
ReplaceableMetadataImpl.  This is only strictly necessary if there are
tracking references to the MDNode.  Make the construction of
ReplaceableMetadataImpl lazy, so that we don't get allocations if we
don't need them.

Since MDNode::isResolved now checks MDNode::isTemporary and
MDNode::NumUnresolved instead of whether a ReplaceableMetadataImpl is
allocated, the internal changes are intrusive (at various internal
checkpoints, isResolved now has a different answer).

However, there should be no real functionality change here; just
slightly lazier allocation behaviour.  The external semantics should be
identical.

llvm-svn: 265279