Fixed a threading race condition where we could crash after calling Debugger::Terminate().

The issue was we have two global variables: one that contains a DebuggerList pointer and one that contains a std::mutex pointer. These get initialized in Debugger::Initialize(), and everywhere that uses these does:

if (g_debugger_list_ptr && g_debugger_list_mutex_ptr)
{
    std::lock_guard<std::recursive_mutex> guard(*g_debugger_list_mutex_ptr);
    // do work while mutex is locked
}

Debugger::Terminate() was deleting and nulling out g_debugger_list_ptr which meant we had a race condition where someone might do the if statement and it evaluates to true, then another thread calls Debugger::Terminate() and deletes and nulls out g_debugger_list_ptr while holding the mutex, and another thread then locks the mutex and tries to use g_debugger_list_ptr. The fix is to just not delete and null out the g_debugger_list_ptr variable.

llvm-svn: 275119