[clang] [clang] add asserts and fixes for enforcing template parameters have valid positions (PR #197598)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This is one of those that seems pretty strange right now.  There is a LOT of work happening in SemaTemplateDeductionDecl that seems to be more than just adding asserts (AND does a ton of work without tests).  Can we break this up into smaller tasks that are clear with what they are accomplishing?

Also, the unsigned->int conversion doesn't really seem justified/clear what is going on.

https://github.com/llvm/llvm-project/pull/197598