[clang-format] Don't wrap struct return types as structs

When using BraceWrapping.AfterClass or BraceWrapping.AfterStruct, the
token annotator relies on the first token of the line to determine if
we're dealing with a struct or class, however, this check is faulty if
it's actually a function with an elaborated struct/class return type, as
is common in C.

This patch skips the check if the brace is already annotated as
FunctionLBrace, in which case we already know it's a function and should
be treated as such.

Fixes https://github.com/llvm/llvm-project/issues/58527

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D146281