Add support for custom op parser/printer hooks to know about result names.

Summary:
This allows the custom parser/printer hooks to do interesting things with
the SSA names.  This patch:

 - Adds a new 'getResultName' method to OpAsmParser that allows a parser
   implementation to get information about its result names, along with
   a getNumResults() method that allows op parser impls to know how many
   results are expected.
 - Adds a OpAsmPrinter::printOperand overload that takes an explicit stream.
 - Adds a test.string_attr_pretty_name operation that uses these hooks to
   do fancy things with the result name.

Reviewers: rriddle!

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76205