[clang-format] Fix requires misannotation with comma (#65908)

clang-format uses a heuristic to determine if a requires() is either a
requires clause or requires expression, based on what is in the
parentheses. Part of this heuristic assumed that a requires clause can
never contain a comma, however this is not the case if said comma is in
the template argument of a type.

This patch allows commas to appear in a requires clause if an angle
bracket `<` has been opened.

Fixes https://github.com/llvm/llvm-project/issues/65904