[flang] Changes related to feedback from pull request flang-compiler/f18#551

 - Changed the name a a function to FindOrDeclareEnclosingEntity
 - Changed that function to return a reference rather than a pointer
 - Polished the setting of the IMPLICIT flag on a declared entity in a
 locality-spec
 - Changed a test to remove an unreported error

Original-commit: flang-compiler/f18@0c1a00d7c2c67064801f9f6fb37c0a26224c0ea4
Reviewed-on: https://github.com/flang-compiler/f18/pull/551
Tree-same-pre-rewrite: false