[flang] Fix accessibility of USEd name in .mod file

If a module specifies default private accessibility, names that have
been use-associated are private by default. This was not reflected in
.mod files.

Differential Revision: https://reviews.llvm.org/D94602

flang/lib/Semantics/mod-file.cpp

@@ -427,6 +427,7 @@ void ModFileWriter::PutUse(const Symbol &symbol) {
     PutGenericName(uses_ << "=>", use);
   }
   uses_ << '\n';
+  PutUseExtraAttr(Attr::PRIVATE, symbol, use);
   PutUseExtraAttr(Attr::VOLATILE, symbol, use);
   PutUseExtraAttr(Attr::ASYNCHRONOUS, symbol, use);
 }

flang/test/Semantics/modfile03.f90

@@ -158,3 +158,21 @@ end
 !  end
 ! end interface
 !end
+
+module m7a
+  real :: x
+end
+!Expect: m7a.mod
+!module m7a
+! real(4)::x
+!end
+
+module m7b
+  use m7a
+  private
+end
+!Expect: m7b.mod
+!module m7b
+! use m7a,only:x
+! private::x
+!end