[flang] Handle ENTRY names in IsPureProcedure() predicate

[klausler]

Summary

Fortran defines an ENTRY point name as being pure if its enclosing
subprogram scope defines a pure procedure.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/include/flang/Evaluate/tools.h

 }
 
 } // namespace Fortran::evaluate
 
 namespace Fortran::semantics {
 
 class Scope;
 
+// If a symbol represents an ENTRY, return the symbol of the main entry
+// point to its subprogram.
+const Symbol *GetMainEntry(const Symbol *);
+
 // These functions are used in Evaluate so they are defined here rather than in
 // Semantics to avoid a link-time dependency on Semantics.
 // All of these apply GetUltimate() or ResolveAssociations() to their arguments.
 bool IsVariableName(const Symbol &);
 bool IsPureProcedure(const Symbol &);
 bool IsPureProcedure(const Scope &);
 bool IsFunction(const Symbol &);
 bool IsFunction(const Scope &);

flang/lib/Evaluate/tools.cpp

const Symbol &GetAssociationRoot(const Symbol &original) {
   if (const auto *details{symbol.detailsIf<AssocEntityDetails>()}) {
     if (const Symbol * root{GetAssociatedVariable(*details)}) {
       return *root;
     }
   }
   return symbol;
 }
 
+const Symbol *GetMainEntry(const Symbol *symbol) {
+  if (symbol) {
+    if (const auto *subpDetails{symbol->detailsIf<SubprogramDetails>()}) {
+      if (const Scope * scope{subpDetails->entryScope()}) {
+        if (const Symbol * main{scope->symbol()}) {
+          return main;
+        }
+      }
+    }
+  }
+  return symbol;
+}
+
 bool IsVariableName(const Symbol &original) {
   const Symbol &symbol{ResolveAssociations(original)};
   if (symbol.has<ObjectEntityDetails>()) {
     return !IsNamedConstant(symbol);
   } else if (const auto *assoc{symbol.detailsIf<AssocEntityDetails>()}) {
     const auto &expr{assoc->expr()};
     return expr && IsVariable(*expr) && !HasVectorSubscript(*expr);
   } else {
     return false;
   }
 }
 
 bool IsPureProcedure(const Symbol &original) {
-  const Symbol &symbol{original.GetUltimate()};
+  // An ENTRY is pure if its containing subprogram is
+  const Symbol &symbol{DEREF(GetMainEntry(&original.GetUltimate()))};
   if (const auto *procDetails{symbol.detailsIf<ProcEntityDetails>()}) {
     if (const Symbol * procInterface{procDetails->interface().symbol()}) {
       // procedure component with a pure interface
       return IsPureProcedure(*procInterface);
     }
   } else if (const auto *details{symbol.detailsIf<ProcBindingDetails>()}) {
     return IsPureProcedure(details->symbol());
   } else if (!IsProcedure(symbol)) {