Revert "[lldb] Use nullptr instead of 0 or NULL (NFC)"

This reverts commit 24d2405588.
Breaks building on Windows:

    ../../lldb/include\lldb/Host/HostNativeThreadBase.h(49,36): error:
        cannot initialize a member subobject of type 'lldb::thread_result_t'
        (aka 'unsigned int') with an rvalue of type 'std::nullptr_t'
      lldb::thread_result_t m_result = nullptr;
                                       ^~~~~~~
    1 error generated.

lldb/source/API/SystemInitializerFull.cpp

@@ -39,7 +39,7 @@ constexpr lldb_private::HostInfo::SharedLibraryDirectoryHelper
 
 #else
 constexpr lldb_private::HostInfo::SharedLibraryDirectoryHelper
-    *g_shlib_dir_helper = nullptr;
+    *g_shlib_dir_helper = 0;
 #endif
 
 using namespace lldb_private;