[clang] [Clang] Do not treat Foo -> const Foo conversion sequences as perfect (PR #148613)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Thanks for the heads up!

I'm looking into it.

Reduced to ```cpp
struct TmsException  {
  int _what;
  const char* what() const;

  template<typename T=int>
  int& what() & { return _what; }
};
int main() {
  TmsException E;
  E.what() = 42;
}```

https://github.com/llvm/llvm-project/pull/148613