[clang-format] Disallow trailing return arrows to be operators

In the following construction:
`template <typename T> requires Foo<T> || Bar<T> auto func() -> int;`

The `->` of the trailing return type was actually considered as an
operator as part of the binary operation in the requires clause, with
the precedence level of `PrecedenceArrowAndPeriod`, leading to fake
parens being inserted in strange locations, that would never be closed.

Fixes one part of https://github.com/llvm/llvm-project/issues/56213
(the rest will probably be in a separate patch)

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D134049