[flang] Fix bogus message on index-names in the presence of associated entities

The semantic analysis of index-names of FORALL statements looks up symbols with
the same name as the index-name.  This is needed to exclude symbols that are
not objects.  But if the symbol found is host-, use-, or construct-associated
with another entity, the check fails.

I fixed this by getting the root symbol of the symbol found and doing the check
on the root symbol.  This required creating a non-const version of
"GetAssociationRoot()".

Differential Revision: https://reviews.llvm.org/D92970