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

lib/Sema/SemaStmt.cpp

if (BeginMemberLookup.empty() != EndMemberLookup.empty()) {
       // This way we prefer a "no viable 'end'" diagnostic over a "i found
       // a 'begin' but ignored it because there was no member 'end'"
       // diagnostic.
       auto BuildNonmember = [&](
           BeginEndFunction BEFFound, LookupResult &Found,
           llvm::function_ref<Sema::ForRangeStatus()> BuildFound,
           llvm::function_ref<Sema::ForRangeStatus()> BuildNotFound) {
         LookupResult OldFound = std::move(Found);
-        Found.clear();
 
         if (Sema::ForRangeStatus Result = BuildNotFound())
           return Result;
 
         switch (BuildFound()) {
         case Sema::FRS_Success:
           return Sema::FRS_Success;