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

[tskeith]

Summary

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

[PeteSteinfeld]

This is an odd quirk of the language.

All builds, tests, and looks good.

flang/lib/Semantics/mod-file.cpp

void ModFileWriter::PutUse(const Symbol &symbol) {
   uses_ << "use " << GetUsedModule(details).name();
   PutGenericName(uses_ << ",only:", symbol);
   // Can have intrinsic op with different local-name and use-name
   // (e.g. `operator(<)` and `operator(.lt.)`) but rename is not allowed
   if (!IsIntrinsicOp(symbol) && use.name() != symbol.name()) {
     PutGenericName(uses_ << "=>", use);
   }
   uses_ << '\n';
+  PutUseExtraAttr(Attr::PRIVATE, symbol, use);
   PutUseExtraAttr(Attr::VOLATILE, symbol, use);
   PutUseExtraAttr(Attr::ASYNCHRONOUS, symbol, use);
 }
 
 // We have "USE local => use" in this module. If attr was added locally
 // (i.e. on local but not on use), also write it out in the mod file.
 void ModFileWriter::PutUseExtraAttr(
     Attr attr, const Symbol &local, const Symbol &use) {

flang/test/Semantics/modfile03.f90

 ! use m6a,only:t2=>t1
 ! interface
 !  subroutine s(x)
 !   import::t2
 !   type(t2)::x
 !  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