[flang] Allow assignment to host association in BLOCK in PURE subprogram

[klausler]

Summary

We need to distinguish BLOCK host association from subprogram host
association when checking assignments in PURE subprograms.
The specific case that is not allowed is an assignment to a variable
from the scope around the PURE subprogram.

flang/include/flang/Semantics/tools.h

 
 bool IsGenericDefinedOp(const Symbol &);
 bool IsDefinedOperator(SourceName);
 std::string MakeOpName(SourceName);
 bool DoesScopeContain(const Scope *maybeAncestor, const Scope &maybeDescendent);
 bool DoesScopeContain(const Scope *, const Symbol &);
 bool IsUseAssociated(const Symbol &, const Scope &);
 bool IsHostAssociated(const Symbol &, const Scope &);
+bool IsHostAssociatedIntoSubprogram(const Symbol &, const Scope &);
 inline bool IsStmtFunction(const Symbol &symbol) {
   const auto *subprogram{symbol.detailsIf<SubprogramDetails>()};
   return subprogram && subprogram->stmtFunction();
 }
 bool IsInStmtFunction(const Symbol &);
 bool IsStmtFunctionDummy(const Symbol &);
 bool IsStmtFunctionResult(const Symbol &);
 bool IsPointerDummy(const Symbol &);

flang/lib/Semantics/assignment.cpp

static bool IsPointerDummyOfPureFunction(const Symbol &x) {
   return IsPointerDummy(x) && FindPureProcedureContaining(x.owner()) &&
       x.owner().symbol() && IsFunction(*x.owner().symbol());
 }
 
 static const char *WhyBaseObjectIsSuspicious(
     const Symbol &x, const Scope &scope) {
   // See C1594, first paragraph.  These conditions enable checks on both
   // left-hand and right-hand sides in various circumstances.
-  if (IsHostAssociated(x, scope)) {
+  if (IsHostAssociatedIntoSubprogram(x, scope)) {
     return "host-associated";
   } else if (IsUseAssociated(x, scope)) {
     return "USE-associated";
   } else if (IsPointerDummyOfPureFunction(x)) {
     return "a POINTER dummy argument of a pure function";
   } else if (IsIntentIn(x)) {
     return "an INTENT(IN) dummy argument";
   } else if (FindCommonBlockContaining(x)) {

flang/lib/Semantics/tools.cpp

 }
 
 bool IsHostAssociated(const Symbol &symbol, const Scope &scope) {
   return DoesScopeContain(
       &GetProgramUnitOrBlockConstructContaining(FollowHostAssoc(symbol)),
       GetProgramUnitOrBlockConstructContaining(scope));
 }
 
+bool IsHostAssociatedIntoSubprogram(const Symbol &symbol, const Scope &scope) {
+  return DoesScopeContain(
+      &GetProgramUnitOrBlockConstructContaining(FollowHostAssoc(symbol)),
+      GetProgramUnitContaining(scope));
+}
+
 bool IsInStmtFunction(const Symbol &symbol) {
   if (const Symbol * function{symbol.owner().symbol()}) {
     return IsStmtFunction(*function);
   }
   return false;
 }
 
 bool IsStmtFunctionDummy(const Symbol &symbol) {