[PATCH] AsmWriter/Bitcode: Support specialized debug nodes

[Adrian Prantl]

A few nitpicky suggestions and questions:

- !MDSubrange(count: 30, lo: 2)
  what about spelling out “low” and “high”?

- do we still need the uniqueID on MDLexicalBlock? Could it be a distinct node instead?

- is it possible to have it print DW_OP-* constants such as in
  !MDExpression(DW_OP_deref, DW_OP_bit_piece, 0, 8)
  ?

Will it still print the old-style comments that testcases tend to match on, or are they obsoleted by the human-readable syntax?

-- adrian