[NPM] Added opt option -print-pipeline-passes.

Added opt option -print-pipeline-passes to print a -passes compatible
string describing the built pass pipeline.

As an example:
$ opt -enable-new-pm=1 -adce -licm -simplifycfg -o /dev/null /dev/null -print-pipeline-passes
verify,function(adce),function(loop-mssa(licm)),function(simplifycfg<bonus-inst-threshold=1;no-forward-switch-cond;no-switch-to-lookup;keep-loops;no-hoist-common-insts;no-sink-common-insts>),verify,BitcodeWriterPass

At the moment this is best-effort only and there are some known
limitations:
- Not all passes accepting parameters will print their parameters
  (currently only implemented for simplifycfg).
- Some ClassName to pass-name mappings are not unique.
- Some ClassName to pass-name mappings are missing (e.g.
  BitcodeWriterPass).

Differential Revision: https://reviews.llvm.org/D108298