Reapply "[Clang][Sema] Diagnose function/variable templates that shadow their...

Reapply "[Clang][Sema] Diagnose function/variable templates that shadow their own template parameters (#78274)" (#79683)

Reapplies #78274 with the addition of a default-error warning
(`strict-primary-template-shadow`) that is issued for instances of
shadowing which were previously accepted prior to this patch.

I couldn't find an established convention for naming diagnostics related
to compatibility with previous versions of clang, so I just used the
prefix `ext_compat_`.