[analyzer] Make StmtDataCollector part of the CloneDetection API

Summary: We probably want to use this useful templates in other pieces of code (e.g. the one from D34329), so we should make this public.

Reviewers: NoQ

Reviewed By: NoQ

Subscribers: cfe-commits, xazax.hun, v.g.vassilev, johannes

Differential Revision: https://reviews.llvm.org/D34880

llvm-svn: 307501