[clan-format] detect function definitions more conservatively

https://reviews.llvm.org/D105964 updated the detection of function
definitions. It had the unfortunate effect to start marking object
definitions with attribute-like macros as function definitions.

This addresses this issue.

Reviewed By: owenpan

Differential Revision: https://reviews.llvm.org/D107269

(cherry picked from commit f6bc6145)