[lldb] Fix building with lldb::once_flag != std::once_flag
This fixes build breakage on e.g. mingw platforms since
ffa2f539ae2a4e79c01b3d54f8b12c63d8781a0c.
The Debugger::ReportWarning function takes a pointer to a
std::once_flag. On many platforms, llvm::once_flag is a typedef
for std::once_flag, but on others, llvm::once_flag is a custom
reimplementation.
