[NFC][Outliner] Delete default ctors for Candidate & OutlinedFunction.

[NFC][Outliner] Delete default ctors for Candidate & OutlinedFunction.

I think it's good practice to avoid having default ctors unless they're really
valid/useful. For OutlinedFunction the default ctor was used to represent a
bail-out value for getOutliningCandidateInfo(), so I changed the API to return
an optional<getOutliningCandidateInfo> instead which seems a tad cleaner.

Differential Revision: https://reviews.llvm.org/D146375