[flang] Address name resolution problems

[klausler]

Summary

Don't emit a bogus error message about a bad forward reference
when it's an IMPORT of a USE-associated symbol; don't ignore
intrinsic functions when USE-associating the contents of a
module when the intrinsic has been explicitly USE'd; allow
PUBLIC or PRIVATE accessibility attribute to be specified
for an enumerator before the declaration of the enumerator.

flang/lib/Semantics/resolve-names.cpp

 bool ScopeHandler::CheckPossibleBadForwardRef(const Symbol &symbol) {
   if (!context().HasError(symbol)) {
     if (auto fwdRef{HadForwardRef(symbol)}) {
+      const Symbol *outer{symbol.owner().FindSymbol(symbol.name())};
+      if (outer && symbol.has<UseDetails>() &&
+          &symbol.GetUltimate() == &outer->GetUltimate()) {
+        // e.g. IMPORT of host's USE association
+        return false;
+      }
       Say(*fwdRef,
           "Forward reference to '%s' is not allowed in the same specification part"_err_en_US,
           *fwdRef)

     }
     for (const auto &[name, symbol] : *useModuleScope_) {
       if (symbol->attrs().test(Attr::PUBLIC) &&
-          !symbol->attrs().test(Attr::INTRINSIC) &&
+          (!symbol->attrs().test(Attr::INTRINSIC) ||
+              symbol->has<UseDetails>()) &&
           !symbol->has<MiscDetails>() && useNames.count(name) == 0) {
         SourceName location{x.moduleName.source};
         if (auto *localSymbol{FindInScope(name)}) {

 bool DeclarationVisitor::Pre(const parser::Enumerator &enumerator) {
   const parser::Name &name{std::get<parser::NamedConstant>(enumerator.t).v};
   Symbol *symbol{FindSymbol(name)};
-  if (symbol) {
+  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.
+    // 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.