[flang] Fix GetHostProcedure() for main program

[klausler]

Summary

It only worked for internal procedures of subprograms,
but must also allow for internal procedures of the
main program. This broke the use of host-associated
implicitly-typed symbols in specification expressions
of internal procedures.

[PeteSteinfeld]

Thanks for the fast action.

All builds, tests, and looks good.

flang/lib/Semantics/resolve-names.cpp

template <typename T> static T &GetInclusiveScope(T &scope) {
   }
   return scope;
 }
 
 Scope &ScopeHandler::InclusiveScope() { return GetInclusiveScope(currScope()); }
 
 Scope *ScopeHandler::GetHostProcedure() {
   Scope &parent{InclusiveScope().parent()};
-  return parent.kind() == Scope::Kind::Subprogram ? &parent : nullptr;
+  switch (parent.kind()) {
+  case Scope::Kind::Subprogram:
+    return &parent;
+  case Scope::Kind::MainProgram:
+    return &parent;
+  default:
+    return nullptr;
+  }
 }
 
 Scope &ScopeHandler::NonDerivedTypeScope() {
   return currScope_->IsDerivedType() ? currScope_->parent() : *currScope_;
 }
 
 void ScopeHandler::PushScope(Scope::Kind kind, Symbol *symbol) {
   PushScope(currScope().MakeScope(kind, symbol));