[flang] Fix crash with alternate returns in modules

[PeteSteinfeld]

Summary

We weren't handling the case of subroutines with alternate returns that
are contained in modules. I changed the code to add an * as the name
of the parameter when creating the .mod file.

[tskeith]

flang/lib/Semantics/mod-file.cpp

833

DEREF is redundant after checking dummyArg is not null.

flang/lib/Semantics/mod-file.cpp

void ModFileWriter::PutSubprogram(const Symbol &symbol) {
   PutAttrs(os, prefixAttrs, std::nullopt, ""s, " "s);
   os << (details.isFunction() ? "function " : "subroutine ");
   os << symbol.name() << '(';
   int n = 0;
   for (const auto &dummy : details.dummyArgs()) {
     if (n++ > 0) {
       os << ',';
     }
+    if (dummy) {
       os << dummy->name();
+    } else {
+      os << "*";
+    }
   }
   os << ')';
   PutAttrs(os, bindAttrs, details.bindName(), " "s, ""s);
   if (details.isFunction()) {
     const Symbol &result{details.result()};
     if (result.name() != symbol.name()) {
       os << " result(" << result.name() << ')';
     }
if (auto &parent{std::get<std::optional<parser::Name>>(parentId.t)}) {
     return std::nullopt;
   }
 }
 
 void SubprogramSymbolCollector::Collect() {
   const auto &details{symbol_.get<SubprogramDetails>()};
   isInterface_ = details.isInterface();
   for (const Symbol *dummyArg : details.dummyArgs()) {
-    DoSymbol(DEREF(dummyArg));
+    if (dummyArg) {
+      DoSymbol(*dummyArg);
+    }
   }
   if (details.isFunction()) {
     DoSymbol(details.result());
   }
   for (const auto &pair : scope_) {
     const Symbol &symbol{*pair.second};
     if (const auto *useDetails{symbol.detailsIf<UseDetails>()}) {
       if (useSet_.count(useDetails->symbol().GetUltimate()) > 0) {

flang/test/Semantics/modfile04.f90

type(t) function f3(x)
     end type
     type(t2) :: x
   end
   function f4() result(x)
     implicit complex(x)
   end
 end
 
+! Module with a subroutine with alternate returns
+module m3
+contains
+  subroutine altReturn(arg1, arg2, *, *)
+    real :: arg1
+    real :: arg2
+  end subroutine
+end module m3
+
 !Expect: m1.mod
 !module m1
 !type::t
 !end type
 !contains
 !pure subroutine s(x,y) bind(c)
 !logical(4),intent(in)::x
 !real(4),intent(inout)::y

 ! end type
 ! type(t2(b=2_4))::x
 ! type(t)::f3
 !end
 !function f4() result(x)
 !complex(4)::x
 !end
 !end
+
+!Expect: m3.mod
+!module m3
+!contains
+!subroutine altreturn(arg1,arg2,*,*)
+!real(4)::arg1
+!real(4)::arg2
+!end
+!end