Implements slice types including `[]const u8` for passing as
formal parameters in DWARF. Breaking on a function accepting
a slice in `gdb` will now yield the same behavior as stage1 and/or
LLVM backend:

```zig
fn sumArrayLens(a: []const u32, b: []const u8) usize {
  return a.len + b.len;
}
```

Both `a` and `b` can now be inspected in the debugger:

```
Breakpoint 1, sumArrayLens (a=..., b=...) at arr.zig:59
(gdb) p a
$1 = {ptr = 0x7fffffff685c, len = 5}
(gdb) p b
$2 = {ptr = 0x7fffffff683d "\252\252\252\\h\377\377\377\177", len = 3}
(gdb)
```