[Sema] Use StringRef::ltrim (NFC)

Author: kazutakahirata

clang/lib/Sema/SemaChecking.cpp

@@ -18376,7 +18376,7 @@ static bool isSetterLikeSelector(Selector sel) {
   if (sel.isUnarySelector()) return false;
 
   StringRef str = sel.getNameForSlot(0);
-  while (!str.empty() && str.front() == '_') str = str.substr(1);
+  str = str.ltrim('_');
   if (str.starts_with("set"))
     str = str.substr(3);
   else if (str.starts_with("add")) {