[clang-format] Don't allow variable decls to have trailing return arrows

The heuristic for determining if an arrow is a trailing return arrow
looks for the auto keyword, along with parentheses. This isn't
sufficient, since it also triggers on variable declarations with an auto
type, and with an arrow operator.

This patch makes sure a function declaration is being parsed, instead of
any other declaration.

Fixes https://github.com/llvm/llvm-project/issues/61469

Reviewed By: HazardyKnusperkeks, owenpan, MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D147377