[AST] Cleanup on getting the underlying decl of using-shdow decl.

[hokein]

Summary

This should be a NFC cleanup. It removes a unnecessary loop to get the underlying
decl, and add an assertion.

The underlying decl of a using-shadow decl is always the original declaration
has been brought into the scope, clang never builds a nested using-shadow
decl (see Sema::BuildUsingShadowDecl).

[sammccall]

This looks OK, though I'm not sure the value of encoding this assumption further.

clang/lib/AST/Decl.cpp

 }
 
 bool NamedDecl::hasLinkage() const {
   return getFormalLinkage() != NoLinkage;
 }
 
 NamedDecl *NamedDecl::getUnderlyingDeclImpl() {
   NamedDecl *ND = this;
-  while (auto *UD = dyn_cast<UsingShadowDecl>(ND))
+  if (auto *UD = dyn_cast<UsingShadowDecl>(ND))
     ND = UD->getTargetDecl();
 
   if (auto *AD = dyn_cast<ObjCCompatibleAliasDecl>(ND))
     return AD->getClassInterface();
 
   if (auto *AD = dyn_cast<NamespaceAliasDecl>(ND))
     return AD->getNamespace();
 

clang/lib/AST/DeclCXX.cpp

 
 void UsingShadowDecl::anchor() {}
 
 UsingShadowDecl::UsingShadowDecl(Kind K, ASTContext &C, DeclContext *DC,
                                  SourceLocation Loc, DeclarationName Name,
                                  BaseUsingDecl *Introducer, NamedDecl *Target)
     : NamedDecl(K, DC, Loc, Name), redeclarable_base(C),
       UsingOrNextShadow(Introducer) {
-  if (Target)
+  if (Target) {
+    assert(!isa<UsingShadowDecl>(Target));
     setTargetDecl(Target);
+  }
   setImplicit();
 }
 
 UsingShadowDecl::UsingShadowDecl(Kind K, ASTContext &C, EmptyShell Empty)
     : NamedDecl(K, nullptr, SourceLocation(), DeclarationName()),
       redeclarable_base(C) {}
 
 UsingShadowDecl *