[flang] Fix bug with IMPORT of USE of USE

[tskeith]

Summary

When a module contained an import of a use-association of a
use-association, we weren't recognizing that the symbols was needed.
The fix is the follow all of the use-associations using GetUltimate().

[klausler]

LGTM

flang/lib/Semantics/mod-file.cpp

for (const Symbol *dummyArg : details.dummyArgs()) {
     DoSymbol(DEREF(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()) > 0) {
+      if (useSet_.count(useDetails->symbol().GetUltimate()) > 0) {
         need_.push_back(symbol);
       }
     }
   }
 }
 
 void SubprogramSymbolCollector::DoSymbol(const Symbol &symbol) {
   DoSymbol(symbol.name(), symbol);

flang/test/Semantics/modfile36.f90

+! RUN: %S/test_modfile.sh %s %t %f18
+
+! Check modfile that contains import of use-assocation of another use-association.
+
+module m1
+  interface
+     subroutine s(x)
+       use, intrinsic :: iso_c_binding, only: c_ptr
+       type(c_ptr) :: x
+     end subroutine
+  end interface
+end module
+!Expect: m1.mod
+!module m1
+! interface
+!  subroutine s(x)
+!   use iso_c_binding, only: c_ptr
+!   type(c_ptr) :: x
+!  end
+! end interface
+!end
+
+module m2
+  use, intrinsic :: iso_c_binding, only: c_ptr
+  interface
+     subroutine s(x)
+       import :: c_ptr
+       type(c_ptr) :: x
+     end subroutine
+  end interface
+end module
+!Expect: m2.mod
+!module m2
+! use iso_c_binding,only:c_ptr
+! interface
+!  subroutine s(x)
+!   import::c_ptr
+!   type(c_ptr)::x
+!  end
+! end interface
+!end