Driver part of debugger tuning.

Adds driver options named -glldb and -gsce to mean -g plus tuning for
lldb and SCE debuggers respectively; the existing -ggdb option does
the same for gdb. Existing options -ggdb0, -ggdb1 etc. unpack into
-ggdb -g<N>.  (There will not be -glldb<N> or -gsce<N> options.) The
tuning gets a target-specific default in the driver, and is passed
into cc1 with the new -debugger-tuning option.

As fallout, fixes where '-gsplit-dwarf -g0' would ignore the -g0 part
on Linux.

Differential Revision: http://reviews.llvm.org/D15651

llvm-svn: 256104