[OpenMP] Refactor the analysis in checkMapClauseBaseExpression using StmtVisitor class.

Summary: This step is the preparation of allowing lvalue in map/motion clause.

Reviewers: ABataev, jdoerfert

Reviewed By: ABataev

Subscribers: guansong, cfe-commits

Tags: #clang, #openmp

Differential Revision: https://reviews.llvm.org/D74970