[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