[BOLT] Use StringRef::contains (NFC) (#139658)

Once we convert EventNames to StringRef, which is cheap, we can call
StringRef::contains without creating a temporary instance of
std::string.

Author: kazutakahirata

bolt/lib/Profile/YAMLProfileReader.cpp

@@ -887,7 +887,7 @@ Error YAMLProfileReader::readProfile(BinaryContext &BC) {
 }
 
 bool YAMLProfileReader::usesEvent(StringRef Name) const {
-  return YamlBP.Header.EventNames.find(std::string(Name)) != StringRef::npos;
+  return StringRef(YamlBP.Header.EventNames).contains(Name);
 }
 
 } // end namespace bolt