[clang] Implement provisional wording for CWG2398 regarding packs (#90820)

This solves some ambuguity introduced in P0522 regarding how
template template parameters are partially ordered, and should reduce
the negative impact of enabling `-frelaxed-template-template-args`
by default.

When performing template argument deduction, a template template
parameter
containing no packs should be more specialized than one that does.

Given the following example:
```C++
template<class T2> struct A;
template<template<class ...T3s> class TT1, class T4> struct A<TT1<T4>>; // #1
template<template<class    T5 > class TT2, class T6> struct A<TT2<T6>>; // #2

template<class T1> struct B;
template struct A<B<char>>;
```

Prior to P0522, candidate `#2` would be more specialized.
After P0522, neither is more specialized, so this becomes ambiguous.
With this change, `#2` becomes more specialized again,
maintaining compatibility with pre-P0522 implementations.

The problem is that in P0522, candidates are at least as specialized
when matching packs to fixed-size lists both ways, whereas before,
a fixed-size list is more specialized.

This patch keeps the original behavior when checking template arguments
outside deduction, but restores this aspect of pre-P0522 matching
during deduction.

---

Since this changes provisional implementation of CWG2398 which has
not been released yet, and already contains a changelog entry,
we don't provide a changelog entry here.