Address comments.

Author: kazutakahirata

llvm/include/llvm/ProfileData/MemProf.h

@@ -1177,6 +1177,10 @@ template <> struct ScalarTraits<memprof::GUIDHex64> {
     Out << format("0x%016" PRIx64, (uint64_t)Val);
   }
   static StringRef input(StringRef Scalar, void *, memprof::GUIDHex64 &Val) {
+    // Reject decimal GUIDs.
+    if (all_of(Scalar, [](char C) { return std::isdigit(C); }))
+      return "use a hexadecimal GUID or a function instead";
+
     uint64_t Num;
     if (Scalar.starts_with_insensitive("0x")) {
       // Accept hexadecimal numbers starting with 0x or 0X.

llvm/unittests/ProfileData/MemProfTest.cpp

@@ -791,7 +791,7 @@ TEST(MemProf, YAMLParserGUID) {
 
   // Verify the entire contents of MemProfData.Records.
   ASSERT_THAT(MemProfData.Records, SizeIs(1));
-  const auto &[GUID, Record] = *MemProfData.Records.begin();
+  const auto &[GUID, Record] = MemProfData.Records.front();
   EXPECT_EQ(GUID, IndexedMemProfRecord::getGUID("_Z3fooi"));
   ASSERT_THAT(Record.AllocSites, SizeIs(1));
   EXPECT_EQ(Record.AllocSites[0].CSId, hashCallStack(CS1));