[BOLT] Use StringRef::consume_front (NFC) (#139432)

Author: kazutakahirata

bolt/lib/Profile/DataReader.cpp

@@ -1035,9 +1035,8 @@ ErrorOr<SampleInfo> DataReader::parseSampleInfo() {
 }
 
 ErrorOr<bool> DataReader::maybeParseNoLBRFlag() {
-  if (ParsingBuf.size() < 6 || ParsingBuf.substr(0, 6) != "no_lbr")
+  if (!ParsingBuf.consume_front("no_lbr"))
     return false;
-  ParsingBuf = ParsingBuf.drop_front(6);
   Col += 6;
 
   if (ParsingBuf.size() > 0 && ParsingBuf[0] == ' ')
@@ -1058,9 +1057,8 @@ ErrorOr<bool> DataReader::maybeParseNoLBRFlag() {
 }
 
 ErrorOr<bool> DataReader::maybeParseBATFlag() {
-  if (ParsingBuf.size() < 16 || ParsingBuf.substr(0, 16) != "boltedcollection")
+  if (!ParsingBuf.consume_front("boltedcollection"))
     return false;
-  ParsingBuf = ParsingBuf.drop_front(16);
   Col += 16;
 
   if (!checkAndConsumeNewLine()) {