[clang] [clang] disallow narrowing when matching template template parameters (PR #124313)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I can't claim to completely understand what is going on here, but most of this seems to be pretty mechanical.  I THINK this seems about right?  2 nits/style concerns, else this looks about right to me.

If someone else could spend some additional time on the tests making sure I didn't miss anything, I would appreciate it.

https://github.com/llvm/llvm-project/pull/124313