[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, 1 insertions, 1 deletions

diff --git a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.h b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.h
index 4be7eb0..898d176 100644
--- a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.h
+++ b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.h
@@ -392,7 +392,7 @@ public:
           *command_output, // Pass nullptr if you don't want the command output
       const Timeout<std::micro> &timeout);
 
-  bool CalculateMD5(const FileSpec &file_spec, uint64_t &low, uint64_t &high);
+  llvm::ErrorOr<llvm::MD5::MD5Result> CalculateMD5(const FileSpec &file_spec);
 
   lldb::DataBufferSP ReadRegister(
       lldb::tid_t tid,