[flang] Fix module file issue with renamed shadowed specific procedures

[klausler]

Summary

A specific procedure in the list of specific procedures associated with
a generic interface needs to be a symbol that is not inadvertently
resolved to its ultimate symbol in another module when it is also
shadowed by a generic interface of the same name.

flang/lib/Semantics/resolve-names.cpp

void InterfaceVisitor::ResolveSpecificsInGeneric(Symbol &generic) {
   for (auto it{range.first}; it != range.second; ++it) {
     const parser::Name *name{it->second.first};
     auto kind{it->second.second};
     const auto *symbol{FindSymbol(*name)};
     if (!symbol) {
       Say(*name, "Procedure '%s' not found"_err_en_US);
       continue;
     }
-    const Symbol &specific{BypassGeneric(*symbol)};
-    const Symbol &ultimate{specific.GetUltimate()};
+    // Subtlety: when *symbol is a use- or host-association, the specific
+    // procedure that is recorded in the GenericDetails below must be *symbol,
+    // not the specific procedure shadowed by a generic, because that specific
+    // procedure may be a symbol from another module and its name unavailable to
+    // emit to a module file.
+    const Symbol &bypassed{BypassGeneric(*symbol)};
+    const Symbol &specific{
+        symbol == &symbol->GetUltimate() ? bypassed : *symbol};
+    const Symbol &ultimate{bypassed.GetUltimate()};
     if (!ultimate.has<SubprogramDetails>() &&
         !ultimate.has<SubprogramNameDetails>()) {
       Say(*name, "'%s' is not a subprogram"_err_en_US);
       continue;
     }
     if (kind == ProcedureKind::ModuleProcedure) {
       if (const auto *nd{ultimate.detailsIf<SubprogramNameDetails>()}) {
         if (nd->kind() != SubprogramKind::Module) {

flang/test/Semantics/modfile50.f90

+! RUN: %python %S/test_modfile.py %s %flang_fc1
+module m1
+  interface foo
+    module procedure foo
+  end interface
+ contains
+  subroutine foo
+  end subroutine
+end module
+module m2
+  use m1, bar => foo
+  interface baz
+    module procedure bar ! must not be replaced in module file with "foo"
+  end interface
+end module
+
+!Expect: m1.mod
+!module m1
+!interface foo
+!procedure::foo
+!end interface
+!contains
+!subroutine foo()
+!end
+!end
+
+!Expect: m2.mod
+!module m2
+!use m1,only:bar=>foo
+!interface baz
+!procedure::bar
+!end interface
+!end