[lldb] Consult summary provider before printing children of root references

When printing the root of a value, if it's a reference its children are unconditionally
printed - in contrast to pointers whose children are only printed if a sufficient
pointer depth is given.

However, the children are printed even when there's a summary provider that says not to.
If a summary provider exists, this change consults it to determine if children should be
printed.

For example, given a variable of type `std::string &`, this change has the following
effect:

Before:

```
(lldb) p string_ref
(std::string &) string_ref = "one two three four five six seven eight nine ten": {
  __r_ = {
    std::__1::__compressed_pair_elem<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep, 0, false> = {
      __value_ = {
         = {
          __l = (__data_ = "one two three four five six seven eight nine ten", __size_ = 48, __cap_ = 64, __is_long_ = 1)
          __s = (__data_ = "@\0p\U00000001\0`\0\00\0\0\0\0\0\0\0@", __padding_ = "\x80t<", __size_ = '\0', __is_long_ = '\x01')
          __r = {
            __words ={...}
          }
        }
      }
    }
  }
}
```

After:

```
(lldb) p string_ref
(std::string &) string_ref = "one two three four five six seven eight nine ten"
```

rdar://73248786

Differential Revision: https://reviews.llvm.org/D151748