[clang-format] Don't annotate left brace of class as FunctionLBrace

The l_brace of class/struct/union was incorrectly annotated as
TT_FunctionLBrace in the presence of attributes. This in turn
would cause the RemoveSemicolon option to remove the semicolon
at the end of the declaration, resulting in invalid code being
generated.

Fixes #61188.

Differential Revision: https://reviews.llvm.org/D145344