[lldb] Port Process::PrintWarning* to use the new diagnostic events

Port the two Process::PrintWarning functions to use the new diagnostic
events through Debugger::ReportWarning. I kept the wrapper function in
the process, but delegated the work to the Module. Consistent with the
current code, the Module ensures the warning is only printed once per
module.

Differential revision: https://reviews.llvm.org/D123698