[flang] Avoid interference between different association variable resoltions

Fix issue 598 and related issues.

Transform resolve-names.cc ConstructVisitor association_ member into a
stack so that different association construct (select-type, select-rank,
change-team, associate) imbrication/succession do not interfere with each
other leading to wrong erronous symbol resolution.

Original-commit: flang-compiler/f18@5781f29ed6d3d235065e4a873a007431f6a48c02
Reviewed-on: https://github.com/flang-compiler/f18/pull/600
Tree-same-pre-rewrite: false