Fix bug in readability-uppercase-literal-suffix

Fixes https://bugs.llvm.org/show_bug.cgi?id=51790. The check triggers
incorrectly with non-type template parameters.

A bisect determined that the bug was introduced here:
https://github.com/llvm/llvm-project/commit/ea2225a10be986d226e041d20d36dff17e78daed

Unfortunately that patch can no longer be reverted on top of the main
branch, so add a fix instead. Add a unit test to avoid regression in
the future.