[flang] Fix spurious "already declared" errors for interfaces

[klausler]

Summary

When a subroutine or function symbol is defined in an INTERFACE
block, it's okay if a symbol of the same name appears in a
scope between the global scope and the scope of the INTERFACE.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/lib/Semantics/resolve-names.cpp

   // Edits an existing symbol created for earlier calls to a subprogram or ENTRY
   // so that it can be replaced by a later definition.
   bool HandlePreviousCalls(const parser::Name &, Symbol &, Symbol::Flag);
-  // Create a subprogram symbol in the current scope and push a new scope.
   void CheckExtantProc(const parser::Name &, Symbol::Flag);
+  // Create a subprogram symbol in the current scope and push a new scope.
   Symbol &PushSubprogramScope(const parser::Name &, Symbol::Flag);
   Symbol *GetSpecificFromGeneric(const parser::Name &);
   SubprogramDetails &PostSubprogramStmt(const parser::Name &);

 void SubprogramVisitor::CheckExtantProc(
     const parser::Name &name, Symbol::Flag subpFlag) {
   if (auto *prev{FindSymbol(name)}) {
-    if (!IsDummy(*prev) && !HandlePreviousCalls(name, *prev, subpFlag)) {
+    if (IsDummy(*prev)) {
+    } else if (inInterfaceBlock() && currScope() != prev->owner()) {
+      // Procedures in an INTERFACE block do not resolve to symbols
+      // in scopes between the global scope and the current scope.
+    } else if (!HandlePreviousCalls(name, *prev, subpFlag)) {
       SayAlreadyDeclared(name, *prev);
     }
   }