DebugInfo: Move the '=' version of -gsimple-template-names to the frontend

Based on feedback from Paul Robinson on 38c09ea2 that the 'mangled' mode
is only useful as an LLVM-developer-internal tool in combination with
llvm-dwarfdump --verify, so demote that to a frontend-only (not driver)
option. The driver support is simply -g{no-,}simple-template-names to
switch on simple template names, without the option to use the mangled
template name scheme there.