[flang] Fix ENUMERATOR declarations in nested scopes

[klausler]

Summary

ENUMERATOR declarations are checking for conflicts between new
enumerator names and names in both the local scope and scopes
that contain it. ENUMERATOR declarations belong to the local
scope and so only a conflict with a name therein is an error.
(In short, use FindInScope rather than FindSymbol.)

flang/lib/Semantics/resolve-names.cpp

bool DeclarationVisitor::Pre(const parser::NamedConstant &x) {
   } else {
     CheckUseError(name);
   }
   return false;
 }
 
 bool DeclarationVisitor::Pre(const parser::Enumerator &enumerator) {
   const parser::Name &name{std::get<parser::NamedConstant>(enumerator.t).v};
-  Symbol *symbol{FindSymbol(name)};
+  Symbol *symbol{FindInScope(name)};
   if (symbol && !symbol->has<UnknownDetails>()) {
     // Contrary to named constants appearing in a PARAMETER statement,
     // enumerator names should not have their type, dimension or any other
     // attributes defined before they are declared in the enumerator statement,
     // with the exception of accessibility.
     // This is not explicitly forbidden by the standard, but they are scalars
     // which type is left for the compiler to chose, so do not let users try to
     // tamper with that.