[clangd] Factor out some helper functions related to heuristic resolution in TargetFinder
Summary:
Two helpers are introduced:
* Some of the logic previously in TargetFinder::Visit*() methods is
factored out into resolveDependentExprToDecls().
* Some of the logic in getMembersReferencedViaDependentName() is
factored out into resolveTypeToRecordDecl().
D82739 will build on this and use these functions in new ways.
Reviewers: hokein
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D83371