clang-format: Fix __attribute__ being treated as decl name.

__attribute__ was treated as the name of a function definition, with the
tokens in parentheses being the parameter list. This formats incorrectly
with AlwaysBreakAfterDefinitionReturnType. Fix it by treating
__attribute__ like decltype.

Patch by Strager Neds, thank you.

llvm-svn: 241439