[lldb] Unify CalculateMD5 return types (#91029)

This is a retake of https://github.com/llvm/llvm-project/pull/90921
which got reverted because I forgot to modify the CalculateMD5 unit test
I had added in https://github.com/llvm/llvm-project/pull/88812

The prior failing build is here:
https://lab.llvm.org/buildbot/#/builders/68/builds/73622
To make sure this error doesn't happen, I ran `ninja
ProcessGdbRemoteTests` and then executed the resulting test binary and
observed the `CalculateMD5` test passed.

# Overview
In my previous PR: https://github.com/llvm/llvm-project/pull/88812,
@JDevlieghere suggested to match return types of the various calculate
md5 functions.

This PR achieves that by changing the various calculate md5 functions to
return `llvm::ErrorOr<llvm::MD5::MD5Result>`.
 
The suggestion was to go for `std::optional<>` but I opted for
`llvm::ErrorOr<>` because local calculate md5 was already possibly
returning `ErrorOr`.

To make sure I didn't break the md5 calculation functionality, I ran
some tests for the gdb remote client, and things seem to work.

# Testing
1. Remote file doesn't exist

![image](https://github.com/llvm/llvm-project/assets/1326275/b26859e2-18c3-4685-be8f-c6b6a5a4bc77)

1. Remote file differs

![image](https://github.com/llvm/llvm-project/assets/1326275/cbdb3c58-555a-401b-9444-c5ff4c04c491)

1. Remote file matches

![image](https://github.com/llvm/llvm-project/assets/1326275/07561572-22d1-4e0a-988f-bc91b5c2ffce)

## Test gaps
Unfortunately, I had to modify
`lldb/source/Plugins/Platform/MacOSX/PlatformDarwinDevice.cpp` and I
can't test the changes there. Hopefully, the existing test suite / code
review from whomever is reading this will catch any issues.

1 files changed, 6 insertions, 7 deletions

diff --git a/lldb/unittests/Process/gdb-remote/GDBRemoteCommunicationClientTest.cpp b/lldb/unittests/Process/gdb-remote/GDBRemoteCommunicationClientTest.cpp
index 6b11ec4..2411139 100644
--- a/lldb/unittests/Process/gdb-remote/GDBRemoteCommunicationClientTest.cpp
+++ b/lldb/unittests/Process/gdb-remote/GDBRemoteCommunicationClientTest.cpp
@@ -595,10 +595,8 @@ TEST_F(GDBRemoteCommunicationClientTest, WriteMemoryTags) {
 
 TEST_F(GDBRemoteCommunicationClientTest, CalculateMD5) {
   FileSpec file_spec("/foo/bar", FileSpec::Style::posix);
-  uint64_t low, high;
-  std::future<bool> async_result = std::async(std::launch::async, [&] {
-    return client.CalculateMD5(file_spec, low, high);
-  });
+  std::future<ErrorOr<MD5::MD5Result>> async_result = std::async(
+      std::launch::async, [&] { return client.CalculateMD5(file_spec); });
 
   lldb_private::StreamString stream;
   stream.PutCString("vFile:MD5:");
@@ -607,11 +605,12 @@ TEST_F(GDBRemoteCommunicationClientTest, CalculateMD5) {
                "F,"
                "deadbeef01020304"
                "05060708deadbeef");
-  ASSERT_TRUE(async_result.get());
+  auto result = async_result.get();
 
   // Server and client puts/parses low, and then high
   const uint64_t expected_low = 0xdeadbeef01020304;
   const uint64_t expected_high = 0x05060708deadbeef;
-  EXPECT_EQ(expected_low, low);
-  EXPECT_EQ(expected_high, high);
+  ASSERT_TRUE(result);
+  EXPECT_EQ(expected_low, result->low());
+  EXPECT_EQ(expected_high, result->high());
 }