[lldb][NFC] Move SBThread::ResumeNewPlan out of the header (llvm#151988)

This *private* method is only defined as a member class of SBThread so
that it may be declared a `friend` of SBError and access a private
constructor of SBError that takes a `Status`, which is an `lldb_private`
object. In other words, the method does not use anything about the class
is belongs to, so it has no business being a member method.

A subsequent patch will need to add a new argument to this class, a
`Process::StopLocker`, which is also another `lldb_private` object. This
is another strong suggestion that this method does not belong on the
header file of a public API, even if at the private visibility level. We
can't forward declare nested types like `Process:StopLocker`, so this
problem is not easily solvable.

This commit moves the method out of the header and into an anon
namespace of the cpp file. It is also made to return a Status instead,
and the private `SBError` constructor is accessed by the SBThread
methods that call it.

(cherry picked from commit 849daa4)

lldb/include/lldb/API/SBThread.h

@@ -255,9 +255,6 @@ class LLDB_API SBThread {
 
   void SetThread(const lldb::ThreadSP &lldb_object_sp);
 
-  SBError ResumeNewPlan(lldb_private::ExecutionContext &exe_ctx,
-                        lldb_private::ThreadPlan *new_plan);
-
   lldb::ThreadSP GetSP() const;
 
   lldb::ExecutionContextRefSP m_opaque_sp;

lldb/source/API/SBThread.cpp

@@ -514,21 +514,14 @@ bool SBThread::GetInfoItemByPathAsString(const char *path, SBStream &strm) {
   return success;
 }
 
-SBError SBThread::ResumeNewPlan(ExecutionContext &exe_ctx,
-                                ThreadPlan *new_plan) {
-  SBError sb_error;
-
+static Status ResumeNewPlan(ExecutionContext &exe_ctx, ThreadPlan *new_plan) {
   Process *process = exe_ctx.GetProcessPtr();
-  if (!process) {
-    sb_error = Status::FromErrorString("No process in SBThread::ResumeNewPlan");
-    return sb_error;
-  }
+  if (!process)
+    return Status::FromErrorString("No process in SBThread::ResumeNewPlan");
 
   Thread *thread = exe_ctx.GetThreadPtr();
-  if (!thread) {
-    sb_error = Status::FromErrorString("No thread in SBThread::ResumeNewPlan");
-    return sb_error;
-  }
+  if (!thread)
+    return Status::FromErrorString("No thread in SBThread::ResumeNewPlan");
 
   // User level plans should be Controlling Plans so they can be interrupted,
   // other plans executed, and then a "continue" will resume the plan.
@@ -541,11 +534,8 @@ SBError SBThread::ResumeNewPlan(ExecutionContext &exe_ctx,
   process->GetThreadList().SetSelectedThreadByID(thread->GetID());
 
   if (process->GetTarget().GetDebugger().GetAsyncExecution())
-    sb_error.ref() = process->Resume();
-  else
-    sb_error.ref() = process->ResumeSynchronous(nullptr);
-
-  return sb_error;
+    return process->Resume();
+  return process->ResumeSynchronous(nullptr);
 }
 
 void SBThread::StepOver(lldb::RunMode stop_other_threads) {