[clang-format] Allow trailing return types in macros

The trailing return type arrow checker verifies that a declaration is
being parsed, however, this isn't true when inside of macros.

It turns out the existence of the auto keyword is enough to make
sure that we're dealing with a trailing return type, and whether we're
in a declaration doesn't matter.

Fixes https://github.com/llvm/llvm-project/issues/47664

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D141811