[clang] Enable C++17 relaxed template template argument matching by default (#89807)

This patch will finally allow us to mark C++17 support in clang as
complete.
    
In order to implement this as a DR and avoid breaking reasonable code
that worked before P0522, this patch implements a provisional resolution
for CWG2398: When deducing template template parameters against each other,
and the argument side names a template specialization, instead of just
deducing A, we deduce a synthesized template template parameter based
on A, but with it's parameters using the template specialization's arguments
as defaults.
    
The driver flag is deprecated with a warning.
    
Fixes https://github.com/llvm/llvm-project/issues/36505