[lldb] Fix that `process signal` completion always returns all signals

[teemperor]

Summary

CompletionRequest::AddCompletion adds the given string as completion of the current
command token. CompletionRequest::TryCompleteCurrentArg only adds it if the current
token is a prefix of the given string. We're using AddCompletion for the process signal
handler which means that process signal SIGIN doesn't get uniquely completed to
process signal SIGINT as we unconditionally add all other signals (such as SIGABRT)
as possible completions. By using TryCompleteCurrentArg we actually do the proper
filtering which will only add SIGINT (as that's the only signal with the prefix 'SIGIN'
in the example above).

[mib]

LGTM!

lldb/source/Commands/CommandObjectProcess.cpp

public:
   HandleArgumentCompletion(CompletionRequest &request,
                            OptionElementVector &opt_element_vector) override {
     if (!m_exe_ctx.HasProcessScope() || request.GetCursorIndex() != 0)
       return;
 
     UnixSignalsSP signals = m_exe_ctx.GetProcessPtr()->GetUnixSignals();
     int signo = signals->GetFirstSignalNumber();
     while (signo != LLDB_INVALID_SIGNAL_NUMBER) {
-      request.AddCompletion(signals->GetSignalAsCString(signo), "");
+      request.TryCompleteCurrentArg(signals->GetSignalAsCString(signo));
       signo = signals->GetNextSignalNumber(signo);
     }
   }
 
 protected:
   bool DoExecute(Args &command, CommandReturnObject &result) override {
     Process *process = m_exe_ctx.GetProcessPtr();
 

lldb/test/API/functionalities/completion/TestCompletion.py

def test_process_signal(self):
         # Test with a running process.
         self.build()
         self.main_source = "main.cpp"
         self.main_source_spec = lldb.SBFileSpec(self.main_source)
         lldbutil.run_to_source_breakpoint(self, '// Break here', self.main_source_spec)
 
         self.complete_from_to('process signal ',
                               'process signal SIG')
+        self.complete_from_to('process signal SIGIN',
+                              'process signal SIGINT')
         self.complete_from_to('process signal SIGA',
                               ['SIGABRT',
                                'SIGALRM'])
 
     def test_ambiguous_long_opt(self):
         self.completions_match('breakpoint modify --th',
                                ['--thread-id',
                                 '--thread-index',