[ADT] Use a C++17 fold expression in hash_combine (NFC) (#159901)

combine() combines hash values with recursion on variadic parameters.

This patch replaces the recursion with a C++17 fold expression:

(combine_data(length, buffer_ptr, buffer_end, get_hashable_data(args)),
   ...);

which expands to:

  combine_data(length, buffer_ptr, buffer_end, get_hashable_data(a));
  combine_data(length, buffer_ptr, buffer_end, get_hashable_data(b));
  combine_data(length, buffer_ptr, buffer_end, get_hashable_data(c));
  :

A key benefit of this change is the unification of the recursive step
and the base case. The argument processing and finalization logic now
exist as straight-line code within a single function.

combine_data now takes buffer_ptr by reference. This is necessary
because the previous assignment pattern:

  buffer_ptr = combine_data(...)

is syntactically incompatible with a fold expression. The new pattern:

  (combine_data(...), ...)

discards return values, so combine_data must update buffer_ptr
directly.

For readability, this patch does the bare minimum to use a fold
expression, leaving further cleanups to subsequent patches.  For
example, buffer_ptr and buffer_end could become member variables, and
several comments that mention recursion still need updating.