Reapply "[Clang][Sema] Diagnose unexpanded packs in the template argument...

Reapply "[Clang][Sema] Diagnose unexpanded packs in the template argument lists of function template specializations" (#76876) (#76915)

This reapplies f034044ad94d6f7ccec13d89f08acac257ed28bb after it was
reverted by 687396b5f4ba0713d103ebd172b308e92eb930cc due to a test
failure in clang-doc.

The test in question declares a partial specialization of a function
template, as well as an explicit specialization of the same function
template. Both declarations are now set as invalid, meaning neither is
emitted by clang-doc.

Since this is the sole test of function template specializations in
clang-doc, I presume the intent is for the partial specialization to
actually be the primary template. Doing so results in the expected
output.