[lldb] Add support for unique target ids (#160736)

### Summary
Add support for unique target ids per Target instance. This is needed
for upcoming changes to allow debugger instances to be shared across
separate DAP instances for child process debugging. We want the IDE to
be able to attach to existing targets in an already runny lldb-dap
session, and having a unique ID per target would make that easier.

Each Target instance will have its own unique id, and uses a
function-local counter in `TargetList::CreateTargetInternal` to assign
incremental unique ids.

### Tests
Added several unit tests to test basic functionality, uniqueness of
targets, and target deletion doesn't affect the uniqueness.
```
bin/lldb-dotest -p TestDebuggerAPI
```

1 files changed, 3 insertions, 0 deletions

diff --git a/lldb/source/Target/Target.cpp b/lldb/source/Target/Target.cpp
index fa98c2460649..e0286c4576ae 100644
--- a/lldb/source/Target/Target.cpp
+++ b/lldb/source/Target/Target.cpp
@@ -139,6 +139,8 @@ private:
 };
 } // namespace
 
+static std::atomic<lldb::user_id_t> g_target_unique_id{1};
+
 template <typename Installer>
 static Status installExecutable(const Installer &installer) {
   if (!installer.m_local_file || !installer.m_remote_file)
@@ -183,6 +185,7 @@ Target::Target(Debugger &debugger, const ArchSpec &target_arch,
       m_source_manager_up(), m_stop_hooks(), m_stop_hook_next_id(0),
       m_latest_stop_hook_id(0), m_valid(true), m_suppress_stop_hooks(false),
       m_is_dummy_target(is_dummy_target),
+      m_target_unique_id(g_target_unique_id++),
       m_frame_recognizer_manager_up(
           std::make_unique<StackFrameRecognizerManager>()) {
   SetEventName(eBroadcastBitBreakpointChanged, "breakpoint-changed");