[flang] Change CheckDefinabilityInPureScope to return bool

Have CheckDefinabilityInPureScope and CheckCopyabilityInPureScope return
false when their checks fail and report errors so that we will be able
to avoid reporting extra errors in those cases.

Original-commit: flang-compiler/f18@305a3470e572161483b0f430514b77ba19f932ea
Reviewed-on: https://github.com/flang-compiler/f18/pull/1050
Tree-same-pre-rewrite: false