[clang-format] Refactor determineStarAmpUsage NFC

There was some duplicate code in determineStarAmpUsage and
determinePlusMinusCaretUsage

Now a `-` or `+` following `;`, `sizeof`, `co_await`, or `delete` is
regarded as a unary operator.

Now a `*` or `&` following `case` is also a unary operator.

Reviewed By: curdeius, MyDeveloperDay, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D121754