Generating unwind information is entangled with supporting exceptions,
even when AsmPrinter explicitly recognizes that the unwind tables are
being generated only as debug information.
Add SupportsDebugUnwindInformation as a workaround for targets which do
not have EH support but which do support unwind information for
debugging. This new option only has an effect when the None EH model
is specified. The option requests that .debug_frame be generated when
debug info is requested.
Add a new AsmPrinterHandler called UnwindStreamer which just ensures the
proper .cfi_sections and .cfi_startproc/.cfi_endproc directives are
emitted when the option is in effect. This duplicates trivial amounts of
DwarfException, but not enough to make factoring it out helpful.
In the future this could be unified/simplified with the existing EH
support if debug handling is made orthogonal to unwind information