While currently available pipeline parsing API (PassBuilder::parsePassPipeline)
is enough for parsing textual representation of a default pipeline,
it becomes insufficient when parsing pipelines for complex custom passes/pass
managers that take nested pipelines. This parsing is being performed by callbacks
(e.g. moudle pipeline parsing callback) which are passed the pass name + array of
pre-parsed pipeline elements. Now after matching the name in callback user has to
process pipeline elements and there is no public API for that.
Whe have methods that take ArrayRef<PipelineElement> - parse*PassPipeline,
but these methods are private.
Just promoting these APIs to public.