[lldb][NFCI] Change parameter type in Process::SetExitStatus

My primary motivation here is actually to change something in
UnixSignals, but this change is a necesary precondition.

I've also updated the documentation and rewritten the log statements to
use `formatv` instead of `printf` (printf-style formatting and
llvm::StringRef don't mix well).

Differential Revision: https://reviews.llvm.org/D157662

Author: bulbazord

lldb/include/lldb/Target/Process.h

@@ -1439,8 +1439,13 @@ class Process : public std::enable_shared_from_this<Process>,
   /// \param[in] exit_status
   ///     The value for the process's return code.
   ///
-  /// \see lldb::StateType
-  virtual bool SetExitStatus(int exit_status, const char *cstr);
+  /// \param[in] exit_string
+  ///     A StringRef containing the reason for exiting. May be empty.
+  ///
+  /// \return
+  ///     Returns \b false if the process was already in an exited state, \b
+  ///     true otherwise.
+  virtual bool SetExitStatus(int exit_status, llvm::StringRef exit_string);
 
   /// Check if a process is still alive.
   ///

lldb/source/Target/Process.cpp

@@ -1053,27 +1053,27 @@ const char *Process::GetExitDescription() {
   return nullptr;
 }
 
-bool Process::SetExitStatus(int status, const char *cstr) {
+bool Process::SetExitStatus(int status, llvm::StringRef exit_string) {
   // Use a mutex to protect setting the exit status.
   std::lock_guard<std::mutex> guard(m_exit_status_mutex);
 
   Log *log(GetLog(LLDBLog::State | LLDBLog::Process));
-  LLDB_LOGF(log, "(plugin = %s status=%i (0x%8.8x), description=%s%s%s)",
-           GetPluginName().data(), status, status, cstr ? "\"" : "",
-           cstr ? cstr : "NULL", cstr ? "\"" : "");
+  LLDB_LOG(log, "(plugin = {0} status = {1} ({1:x8}), description=\"{2}\")",
+           GetPluginName(), status, exit_string);
 
   // We were already in the exited state
   if (m_private_state.GetValue() == eStateExited) {
-    LLDB_LOGF(log,
-             "(plugin = %s) ignoring exit status because state was already set "
-             "to eStateExited",
-             GetPluginName().data());
+    LLDB_LOG(
+        log,
+        "(plugin = {0}) ignoring exit status because state was already set "
+        "to eStateExited",
+        GetPluginName());
     return false;
   }
 
   m_exit_status = status;
-  if (cstr)
-    m_exit_string = cstr;
+  if (!exit_string.empty())
+    m_exit_string = exit_string.str();
   else
     m_exit_string.clear();