gallium/radeon: send LLVM diagnostics as debug messages

gallium/radeon: send LLVM diagnostics as debug messages

Diagnostics sent during code generation and the every error message reported
by LLVMTargetMachineEmitToMemoryBuffer are disjoint reporting mechanisms. We
take care of both and also send an explicit message indicating failure at the
end, so that log parsers can more easily tell the boundary between shader
compiles.

Removed an fprintf that could never be triggered.

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>