[clang-format] Allow decltype in requires clause (#78847)

If clang-format is not sure whether a `requires` keyword starts a
requires clause or a requires expression, it looks ahead to see if any
token disqualifies it from being a requires clause. Among these tokens
was `decltype`, since it fell through the switch.

This patch allows decltype to exist in a require clause.

I'm not 100% sure this change won't have repercussions, but that just
means we need more test coverage!

Fixes https://github.com/llvm/llvm-project/issues/78645