[clang] [clang] check deduction consistency when partial ordering function templates (PR #100692)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Hello, sorry I missed your earlier notification.

Your test case further reduces to: https://godbolt.org/z/q6ndebaeW
```C++
template <char> struct Traits { using Type = Traits; };

template <char... ArgType> void FillVRegs(typename Traits<ArgType>::Type...);
template <char FirstArgType> void FillVRegs(typename Traits<FirstArgType>::Type);

void foo() { FillVRegs<'A'>(Traits<'A'>()); }
```

I believe Clang and GCC are correct to reject this, we can't partial order `FillVRegs` because we can't deduce anything from the function arguments.

https://github.com/llvm/llvm-project/pull/100692