Introduce the DeclReferenceMap class inside the AST library.
DeclReferenceMap (similar to ParentMap) is a helper class for mapping Decls to the AST nodes that reference them.
A client will initialize it by passing an ASTContext to its constructor and later use it to iterate over
the references of a Decl.
References are mapped and retrieved using the primary declaration (Decl::getPrimaryDecl()) of a particular Decl.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@74801 91177308-0d34-0410-b5e6-96231b3b80d8