[NFC] Clean up doc comment and implementation for Module::isSubModuleOf.

[varungandhi-apple]

Updated commit message with reviewer information and revision link.

[aprantl]

clang/lib/Basic/Module.cpp

175–176

for (const Module *Parent = this; Parent; Parent = Parent->Parent) {
  if (Parent == Other)
   return true;
}

?

[varungandhi-apple]

clang/lib/Basic/Module.cpp

175–176

That seems confusing... I would like to avoid giving a local variable the same name as an instance member. I can rename This to Current if that makes it clearer?

[varungandhi-apple]

(Could you land the patch if it looks good? I don't have commit access.)

[varungandhi-apple]

Thanks!

clang/include/clang/Basic/Module.h

bool isAvailable(const LangOptions &LangOpts,
                    const TargetInfo &Target,
                    Requirement &Req,
                    UnresolvedHeaderDirective &MissingHeader,
                    Module *&ShadowingModule) const;
 
   /// Determine whether this module is a submodule.
   bool isSubModule() const { return Parent != nullptr; }
 
-  /// Determine whether this module is a submodule of the given other
-  /// module.
+  /// Check if this module is a (possibly transitive) submodule of \p Other.
+  ///
+  /// The 'A is a submodule of B' relation is a partial order based on the
+  /// the parent-child relationship between individual modules.
+  ///
+  /// Returns \c false if \p Other is \c nullptr.
   bool isSubModuleOf(const Module *Other) const;
 
   /// Determine whether this module is a part of a framework,
   /// either because it is a framework module or because it is a submodule
   /// of a framework module.
   bool isPartOfFramework() const {
     for (const Module *Mod = this; Mod; Mod = Mod->Parent)
       if (Mod->IsFramework)

clang/lib/Basic/Module.cpp

if (!Current->MissingHeaders.empty()) {
       MissingHeader = Current->MissingHeaders.front();
       return false;
     }
   }
 
   llvm_unreachable("could not find a reason why module is unavailable");
 }
 
 bool Module::isSubModuleOf(const Module *Other) const {
-  const Module *This = this;
-  do {
-    if (This == Other)
+  for (auto *Parent = this; Parent; Parent = Parent->Parent) {
+    if (Parent == Other)
       return true;
-
-    This = This->Parent;
-  } while (This);
-
+  }
   return false;
 }
 
 const Module *Module::getTopLevelModule() const {
   const Module *Result = this;
   while (Result->Parent)
     Result = Result->Parent;