[memprof] Use front instead of begin in a unit test (NFC)

"front" allows us to drop a dereference.

Author: kazutakahirata

llvm/unittests/ProfileData/MemProfTest.cpp

@@ -747,7 +747,7 @@ TEST(MemProf, YAMLParser) {
 
   // 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, 0xdeadbeef12345678ULL);
   ASSERT_THAT(Record.AllocSites, SizeIs(2));
   EXPECT_EQ(Record.AllocSites[0].CSId, hashCallStack(CS1));