[flang] Fix a crash with IMPORT in BLOCK (issue 604)

Only call scope.name() if the scope has a symbol (Block, Global,
Forall and ImpliedDo kind of scopes do not have a symbol).

Original-commit: flang-compiler/f18@81c6b67
Reviewed-on: flang-compiler/f18#634
Tree-same-pre-rewrite: false

flang/lib/semantics/resolve-names.cc

@@ -5297,7 +5297,7 @@ void ResolveNamesVisitor::CheckImports() {
     // C8102: all entities in host must not be hidden
     for (const auto &pair : scope.parent()) {
       auto &name{pair.first};
-      if (name != scope.name()) {
+      if (!scope.GetSymbol() || name != scope.name()) {
         CheckImport(*prevImportStmt_, name);
       }
     }

flang/lib/semantics/scope.h

@@ -89,6 +89,8 @@ class Scope {
   const Symbol *GetSymbol() const;
   const Scope *GetDerivedTypeParent() const;
 
+  // It is only safe to call name() for kind of scopes for which GetSymbol
+  // will return a symbol (e.g, it will die if the scope is a Block).
   const SourceName &name() const { return DEREF(GetSymbol()).name(); }
 
   /// Make a scope nested in this one

flang/test/semantics/resolve28.f90

@@ -58,6 +58,12 @@ function f
   import, all
 end
 
+subroutine sub2()
+  block
+    import, all !OK
+  end block
+end
+
 !ERROR: IMPORT is not allowed in a main program scoping unit
 import
 end