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.

llvm-svn: 74801