[memprof] Use unknown_function error type for missing functions

[tejohnson]

Summary

Switch the error type when a function is not found in the memprof
profile to unknown_function. This gives compatibility with normal PGO
function matching, and also prevents issuing large numbers of additional
matching errors since pgo-warn-missing-function is off by default.

[snehasish]

lgtm

llvm/lib/ProfileData/InstrProfReader.cpp

 Expected<memprof::MemProfRecord>
 IndexedInstrProfReader::getMemProfRecord(const uint64_t FuncNameHash) {
   // TODO: Add memprof specific errors.
   if (MemProfRecordTable == nullptr)
     return make_error<InstrProfError>(instrprof_error::invalid_prof,
                                       "no memprof data available in profile");
   auto Iter = MemProfRecordTable->find(FuncNameHash);
   if (Iter == MemProfRecordTable->end())
-    return make_error<InstrProfError>(instrprof_error::hash_mismatch,
-                                      "memprof record not found for hash " +
-                                          Twine(FuncNameHash));
+    return make_error<InstrProfError>(
+        instrprof_error::unknown_function,
+        "memprof record not found for function hash " + Twine(FuncNameHash));
 
   // Setup a callback to convert from frame ids to frame using the on-disk
   // FrameData hash table.
   memprof::FrameId LastUnmappedFrameId = 0;
   bool HasFrameMappingError = false;
   auto IdToFrameCallback = [&](const memprof::FrameId Id) {
     auto FrIter = MemProfFrameTable->find(Id);
     if (FrIter == MemProfFrameTable->end()) {

llvm/unittests/ProfileData/InstrProfTest.cpp

const IndexedMemProfRecord IndexedMR = makeRecord(
       });
   // We skip adding the frame mappings here unlike the test_memprof unit test
   // above to exercise the failure path when getMemProfRecord is invoked.
   Writer.addMemProfRecord(/*Id=*/0x9999, IndexedMR);
 
   auto Profile = Writer.writeBuffer();
   readProfile(std::move(Profile));
 
+  // Missing frames give a hash_mismatch error.
   auto RecordOr = Reader->getMemProfRecord(0x9999);
-  EXPECT_THAT_ERROR(RecordOr.takeError(), Failed());
+  ASSERT_TRUE(
+      ErrorEquals(instrprof_error::hash_mismatch, RecordOr.takeError()));
+
+  // Missing functions give a unknown_function error.
+  RecordOr = Reader->getMemProfRecord(0x1111);
+  ASSERT_TRUE(
+      ErrorEquals(instrprof_error::unknown_function, RecordOr.takeError()));
 }
 
 TEST_F(InstrProfTest, test_memprof_merge) {
   Writer.addRecord({"func1", 0x1234, {42}}, Err);
 
   InstrProfWriter Writer2;
   ASSERT_THAT_ERROR(Writer2.mergeProfileKind(InstrProfKind::MemProf),
                     Succeeded());