Annotate LookupResult::clear() as LLVM_ATTRIBUTE_REINITIALIZES to silence bugprone-use-after-move after rC342925

[rsmith]

I think the check is wrong here. We need this clear call to reinitialize the object after moving from it. Maybe that function should be annotated as reinitializing the object instead?

[MaskRay]

Depends on D52451 (adding LLVM_ATTRIBUTE_REINITIALIZES)

[MaskRay]

Update title with arc diff --edit --verbatim

include/clang/Sema/Lookup.h

public:
   /// Make these results show that the name was found in
   /// different contexts and a tag decl was hidden by an ordinary
   /// decl in a different context.
   void setAmbiguousQualifiedTagHiding() {
     setAmbiguous(AmbiguousTagHiding);
   }
 
   /// Clears out any current state.
-  void clear() {
+  LLVM_ATTRIBUTE_REINITIALIZES void clear() {
     ResultKind = NotFound;
     Decls.clear();
     if (Paths) deletePaths(Paths);
     Paths = nullptr;
     NamingClass = nullptr;
     Shadowed = false;
   }