BitcodeWriter: Emit distinct nodes before uniqued nodes

When an operand of a distinct node hasn't been read yet, the reader can
use a DistinctMDOperandPlaceholder.  This is much cheaper than forward
referencing from a uniqued node.  Change
ValueEnumerator::organizeMetadata to partition distinct nodes and
uniqued nodes to reduce the overhead of cycles broken by distinct nodes.

Mehdi measured this for me; this removes most of the RAUW from the
importing step of -flto=thin, even after a WIP patch that removes
string-based DITypeRefs (introducing many more cycles to the metadata
graph).

llvm-svn: 267276