[flang] Silence bogus error on use after IMPORT

When a scope uses an explicit IMPORT statement to import a
symbol from the scope's host, it should not emit a bogus error
message later if that symbol is used in a specification construct.
The code that checks for imports being hidden by local declarations
was not allowing for the presence of host association (or USE)
indirection symbols in the local scope.  Fix by using GetUltimate()
before checking for the hidden symbol.

Differential Revision: https://reviews.llvm.org/D118747